errno: -1 is EPERM, and four places were saying it by accident

Ctrl-C killed the shell. consoleread() breaks off the read in progress when
SIGINT arrives and returned -1 to do it -- and -1 is -EPERM by the time it
reaches a program, so every program that handles SIGINT by carrying on saw a
permanent error instead of an interrupted call:

    zsh: error on TTY read: operation not permitted
    schinitd: restarting os.schos.console-shell

and the next Ctrl-C did it to the replacement. EINTR now, which is what a
signal-interrupted read is required to give. Verified: Ctrl-C an interruptible
command, shell survives, zero restarts.

Same mistake, same file family: link, unlink, rename and renameat returned -1
on every error path. Callers branch on errno -- `if (unlink(p) < 0 && errno !=
ENOENT)` is the standard shape, and against EPERM it always complains -- which
is why zsh reported failures for things that had worked. They return ENOENT,
EEXIST, EXDEV, EISDIR, ENOTEMPTY and EFAULT now. About ninety other `return -1`s
remain in sys_fs.c; these four are done because these four were what a program
tripped over. /dev/mem's unknown minors return ENODEV rather than -1 too.

/dev/urandom was a linear congruential generator seeded with a **constant**, so
every boot produced the same bytes in the same order -- not weak, the same. A
key, a nonce, a temporary filename: shared with every SchOS machine that had
ever run. Seeded at boot from RDRAND where the CPU has it, from the cycle
counter and the wall clock where it does not, and re-stirred per byte. Still an
LCG and still not a CSPRNG; what changed is the part that was dangerous rather
than merely poor.

**Not fixed, and recorded as such:** rename() still will not replace an existing
target, so zsh's history save fails on every exit after the first. An attempt to
unlink the target first was reverted -- the directory is an HTree, dirunlink
followed by dirlink of the same name did not behave, and it panicked in
filewrite with the old file already gone. Losing both files is worse than
refusing, so it refuses until the HTree's slot reuse is understood.

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

docs/architecture/devices.md

@@ -250,6 +250,26 @@ existing mechanisms for keeping that true mechanically rather than by intention.
 
 ---
 
+## `/dev/random` and `/dev/urandom`
+
+Seeded once at boot, from `RDRAND` where the CPU has it and from the cycle
+counter mixed with the wall clock where it does not. A verbose boot says which:
+
+```
+random: seeded from RDRAND
+```
+
+That is worth a section only because of what it replaced. It was a linear
+congruential generator seeded with a **constant**, so every boot produced the
+same bytes in the same order — not weak, *the same*. A program asking for a
+key, a nonce, a temporary filename or a hash seed got one it shared with every
+other SchOS machine that had ever run.
+
+**It is still not a CSPRNG and does not pretend to be.** It is an LCG, re-stirred
+from the cycle counter on each byte so two readers do not continue one
+sequence. What changed is the property that made it dangerous rather than
+merely poor.
+
 ## What it does not do
 
 - **It is a flat list, not a device tree.** There is no parent/child nesting,