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.

include/uaccess.h

@@ -11,13 +11,13 @@
 //      unrelated physical page at the next 4K boundary. Any multi-byte
 //      access through it must stay within a single page.
 //
-// These helpers encapsulate both rules. Because Schunix (like old x86
+// These helpers encapsulate both rules. Because SchOS (like old x86
 // without SMAP) keeps the user page table active in kernel mode, a raw
 // user virtual address is directly dereferenceable once the whole range
 // has been validated — that is what user_ptr() hands back, and why the
 // result is safe across page boundaries where a uva2ka() pointer is not.
-#ifndef SCHUNIX_UACCESS_H
-#define SCHUNIX_UACCESS_H
+#ifndef SCHOS_UACCESS_H
+#define SCHOS_UACCESS_H
 
 #include "types.h"
 
@@ -46,4 +46,4 @@ int copy_to_user(addr_t uaddr, const void *src, uint64 n);
 // or exceeds USER_STR_MAX.
 long strlen_user(addr_t uaddr);
 
-#endif // SCHUNIX_UACCESS_H
+#endif // SCHOS_UACCESS_H