Rename Schunix to SchOS; the kernel becomes SchIFT

Removes the "-unix" element and with it any trademark exposure to The
Open Group.

The split follows the ecosystem layout: SchOS is the system, and the
kernel has its own name. Kernel-identity strings are the only places
that take SchIFT -- uname(2) sysname, /proc/version, and the two licence
sentences that say "kernel" outright. That is what every Unix does here:
`uname -s` on Linux reports the kernel, not the distribution. Everything
a user reads as the product -- panic screen, boot menu, error prefixes,
docs, LICENSE, NOTICE -- is SchOS.

schunix.img is schos.img, the logo assets and the musl TLS patch are
renamed, and __SCHUNIX__ is __SCHOS__ (the define and the patch that
consumes it moved together).

Two things the rename broke, both found by the bootcheck suite:

  - shmtest wrote "SCHUNIX-MMAP-OK" with a hardcoded length of 15 and
    compared it back by hand-counted offsets (m[3]=='U', m[14]=='K').
    The shorter marker left the write running two bytes past the literal
    and the compare reading past the data, failing a file-backed mmap
    that was working correctly. Now written and compared through one
    constant with sizeof.
  - bootcheck asserted a 31-byte file after appending to
    "hello-from-schunix"; the shorter string makes it 29.

Both are the same mistake: a string literal whose length is written out
somewhere else. Full suite green -- base, Tty, Ps, Sock, Fb, Usb, Evdev.

kernel/fs/exec.c

@@ -11,7 +11,7 @@
 #include "errno.h"
 
 // Binaries with entry points at 0x100000+ are musl/Linux-ABI.
-// Schunix-native binaries (init) are linked at 0x4000.
+// SchOS-native binaries (init) are linked at 0x4000.
 #define LINUX_ABI_ENTRY_THRESHOLD 0x100000UL
 extern int linux_syscall_map[];
 extern int linux_syscall_map_size;
@@ -158,7 +158,7 @@ exec(char *path, char **argv)
   }
 
   // Auto-select syscall ABI: musl binaries have entry >= 0x100000;
-  // Schunix-native binaries (init) are linked at 0x4000.
+  // SchOS-native binaries (init) are linked at 0x4000.
   if (elf.entry >= LINUX_ABI_ENTRY_THRESHOLD) {
     myproc()->syscall_map = linux_syscall_map;
     myproc()->map_size    = linux_syscall_map_size;

kernel/fs/file.c

@@ -152,7 +152,7 @@ fileread(struct file *f, char *addr, int n)
     }
 
     if((f->ip->rwmode & 2) == 0){
-      cprintf("Schunix: permission denied (read)\n");
+      cprintf("SchOS: permission denied (read)\n");
       iunlock(f->ip);
       return -1;
     }
@@ -208,7 +208,7 @@ filewrite(struct file *f, char *addr, int n)
     }
 
     if((f->ip->rwmode & 1) == 0){
-      cprintf("Schunix: permission denied (write)\n");
+      cprintf("SchOS: permission denied (write)\n");
       iunlock(f->ip);
       return -1;
     }

kernel/fs/procfs.c

@@ -133,7 +133,7 @@ static void sbputi(struct sb *s, int64 v)
 // Linux's /proc/<pid>/stat, as far as anything here reads it.
 //
 // The field order is not negotiable: ps counts fields by position, so a
-// missing one silently shifts everything after it. Fields Schunix does not
+// missing one silently shifts everything after it. Fields SchOS does not
 // track are zero, which is what Linux itself reports for things a given
 // process has never done.
 static char

kernel/fs/schfs/kmount.c

@@ -532,7 +532,7 @@ schfs_kroot(void)
 /* Phase 2 has no VFS integration -- SchFS files are not reachable      */
 /* through open(2) yet -- so this is how the mount is observable from   */
 /* userland at all. It reports the volume and lists the root directory, */
-/* which is enough to confirm from inside a running Schunix that the    */
+/* which is enough to confirm from inside a running SchOS that the    */
 /* kernel really did parse an image mkschfs wrote.                      */
 /* ------------------------------------------------------------------ */