kernel: processes have an environment now, and zsh is the shell

Porting zsh found four defects in the kernel and one in the filesystem. None
of them were about zsh; all of them had been there the whole time.

**exec threw the environment away.** kernel/fs/exec.c wrote a single zero for
envp with the comment "(no environment)" -- not yet, but ever, for every
process. musl has been passing a perfectly good envp to execve (Linux 59, which
SchOS answers) and it was never read. So $TERM was unset, $PATH was whatever
each program invented, and a variable exported in a shell did not survive the
exec meant to carry it. exec() takes an envp, sys_exec reads argument three,
and schinit manifests grew an Environment key. The userland two-argument
exec(path, argv) stays as a wrapper that passes null explicitly -- the raw stub
is execve now, so a short caller cannot leave %rdx holding a stray pointer.

**Which overflowed the kernel stack.** Three pointer arrays, ~1.8 KB, on a
4096-byte stack, under exec -> loaduvm -> readi -> SchFS -> btree. Pid 1 went
down on the first exec. The canary caught it and named the process, which is
what it is for. They live in one kalloc'd page now.

**waitpid returned a bare -1** for both "no children" and "interrupted". musl
reads [-4095,-1] as -errno, so userspace saw EPERM; zsh printed "wait failed:
operation not permitted" and abandoned the rest of /etc/zsh/zshrc. -ECHILD and
-EINTR now. Third time a bare -1 has cost real time -- on a system with no
permission model, EPERM should be read as "a syscall returned -1".

**NVMA was 64.** musl's allocator takes a mapping per size class and does not
release them eagerly, so compinit ran out of slots and got -1, which is EPERM,
which malloc turns into NULL, which zsh reports as "fatal error: out of memory"
-- on a machine with 1000 MB free. 256 slots, and -ENOMEM.

**And a small write to SchFS costs 58 ms.** 200 writes to one open file take
11.75 s; the same to /dev/null take 0.14 s. That is the design stated at the
top of kmount.c -- one transaction group per operation, and a commit is two
flushes and an uberblock -- not a bug. But compdump writes its digest with
thousands of separate prints, so it takes minutes and looks like a hang.
compinit -D, which skips the dump, finishes in 3.6 s. This one is a decision
rather than a fix, so it is written up and left: /etc/zsh/zshrc keeps compinit
switched off and explains exactly why, with the measurements.

zsh runs as /bin/zsh and is the console shell. ZLE works -- arrow keys recall
history through terminfo, $COLUMNS reads the real 160 because TIOCGWINSZ was
fixed, colour and prompt expansion work, startup is 0.14 s. mksh keeps /bin/sh,
so schinit's no-jobs fallback is a different program from the one that could
have failed.

Also fixed while here: AT_CLKTCK was the literal 100 and is USER_HZ; the
extract rule in four ports could move a source tree inside itself when make
re-ran it, which is how zsh's configure came to write modfile=zsh-5.9/... into
config.modules and fail three steps later.

schfs-check passes. Full writeup in docs/history/kernel.md section 14.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2

kernel/sys/sys_fs.c

@@ -726,8 +726,8 @@ bad_unlink: iunlockput(dp); end_op(); return -1;
 
 addr_t sys_exec(void)
 {
-    char *path, *argv[MAXARG];
-    addr_t uargv, uarg;
+    char *path, *argv[MAXARG], *envp[MAXENV];
+    addr_t uargv, uenvp, uarg;
     if (argstr(0, &path) < 0 || argaddr(1, &uargv) < 0) return -EFAULT;
     memset(argv, 0, sizeof(argv));
     for (int i = 0; ; i++) {
@@ -736,7 +736,24 @@ addr_t sys_exec(void)
         if (!uarg) { argv[i] = 0; break; }
         if (fetchstr(uarg, &argv[i]) < 0) return -EFAULT;
     }
-    return exec(path, argv);
+
+    // The third argument. This is Linux's execve(2), so musl has been passing
+    // an environment here all along -- it was simply never read, and every
+    // program on the system started with an empty one.
+    //
+    // A null envp is legal and means exactly that. It is also what the
+    // first-party exec() wrapper passes, so old callers keep their old
+    // behaviour without knowing about this.
+    memset(envp, 0, sizeof(envp));
+    if (argaddr(2, &uenvp) == 0 && uenvp) {
+        for (int i = 0; ; i++) {
+            if (i >= NELEM(envp)) return -E2BIG;
+            if (fetchaddr(uenvp + sizeof(addr_t) * i, &uarg) < 0) return -EFAULT;
+            if (!uarg) { envp[i] = 0; break; }
+            if (fetchstr(uarg, &envp[i]) < 0) return -EFAULT;
+        }
+    }
+    return exec(path, argv, envp);
 }
 
 // ftruncate/truncate both grow and shrink. Growing used to be quietly dropped