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/sys/linux_abi.c

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: Apache-2.0
-// Linux x86_64 syscall number -> Schunix syscall number translation.
+// Linux x86_64 syscall number -> SchOS syscall number translation.
 // Enabled per process by sys_enable_linux_abi (see kernel/sys/sys_compat.c).
 //
 // This file used to be the second half of a two-place registration scheme: a
@@ -16,7 +16,7 @@
 
 #define LINUX_NSYSCALL NSYSCALLS
 
-// Index: Linux x86_64 syscall number. Value: Schunix SYS_* number, or 0 for
+// Index: Linux x86_64 syscall number. Value: SchOS SYS_* number, or 0 for
 // "not implemented" (syscall() then returns -ENOSYS).
 //
 // Syscalls declared NO_LINUX in table.inc all land on the single scratch index

kernel/sys/sys_compat.c

@@ -114,8 +114,8 @@ sys_uname(void)
     struct utsname *u = (struct utsname*)user_ptr(ubuf, sizeof(*u));
     if (!u) return -EFAULT;
     memset(u, 0, sizeof(*u));
-    memmove(u->sysname,    "Schunix",  8);
-    memmove(u->nodename,   "schunix",  8);
+    memmove(u->sysname,    "SchIFT",   7);
+    memmove(u->nodename,   "schos",    6);
     memmove(u->release,    "0.1.0",    6);
     memmove(u->version,    "#1",       3);
     memmove(u->machine,    "x86_64",   7);

kernel/sys/sys_fs.c

@@ -18,7 +18,7 @@
 // -----------------------------------------------------------------------
 // Linux-ABI-compatible struct stat (x86_64 "struct stat" from
 // musl 1.2.5 bits/stat.h) — musl/toybox/dash parse st_mode directly
-// out of this exact byte layout, which bears no resemblance to Schunix's
+// out of this exact byte layout, which bears no resemblance to SchOS's
 // own native `struct stat` (stat.h). Without this, every Linux-ABI process
 // reading stat() results (e.g. ls checking S_ISDIR to decide whether to
 // list a directory or just print its name) gets garbage.
@@ -825,7 +825,7 @@ sys_getdents64(void)
 }
 
 // -----------------------------------------------------------------------
-// Custom SchunixFS ops
+// Custom SchOSFS ops
 // -----------------------------------------------------------------------
 
 addr_t sys_delete(void)

kernel/sys/table.inc

@@ -1,7 +1,7 @@
 // Segregated Kernel -- central syscall table. ONE line per syscall.
 //
 // Format: SYSCALL(nr, linux_nr, name)
-//   nr        Schunix native syscall number
+//   nr        SchOS native syscall number
 //   linux_nr  Linux x86_64 syscall number, or NO_LINUX if none
 //   name      implemented as sys_<name>() in kernel/sys/sys_*.c
 //
@@ -120,7 +120,7 @@ SYSCALL( 60,         16, ioctl)
 // translation answered "bad syscall" before the stub could answer ENOSYS.
 // Now that they do something they need the numbers programs actually use.
 //
-// `shutdown` is deliberately not the name here: Schunix already has a native
+// `shutdown` is deliberately not the name here: SchOS already has a native
 // shutdown() that powers the machine off (syscall 32), and two syscalls
 // cannot share a sys_ function name. Linux 48 maps to sockshutdown instead.
 SYSCALL( 40,         41, socket)