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.

cmd/shmtest/main.c

@@ -77,19 +77,29 @@ main(void)
     check(p[100] == 'X', "and so is one further into the segment");
 
     // --- file-backed mmap --------------------------------------------------
+    // Written and compared through one constant, and never by hand-counted
+    // offsets. Those offsets used to spell out SCHUNIX-MMAP-OK a character at a
+    // time -- m[3]=='U' and m[14]=='K' -- with the length 15 written out
+    // separately. Renaming the marker to SCHOS shortened it to 13 and left both
+    // behind: the write ran two bytes off the end of the literal and the
+    // compare read past the data, which failed a file-backed mmap that was
+    // working correctly.
+    static const char MARK[] = "SCHOS-MMAP-OK";
+    const int MARKLEN = (int)(sizeof(MARK) - 1);
+
     int fd = open("/tmp/shmtest.dat", O_CREATE | O_RDWR);
     check(fd >= 0, "created a file to map");
     if (fd >= 0) {
-        write(fd, "SCHUNIX-MMAP-OK", 15);
+        write(fd, MARK, MARKLEN);
         close(fd);
     }
 
     fd = open("/tmp/shmtest.dat", O_RDONLY);
     if (fd >= 0) {
         char *m = mmap(0, 4096, PROT_READ, MAP_PRIVATE, fd, 0);
-        int ok = ((long)m > 0) &&
-                 m[0]=='S' && m[1]=='C' && m[2]=='H' && m[3]=='U' &&
-                 m[8]=='M' && m[13]=='O' && m[14]=='K';
+        int ok = ((long)m > 0);
+        for (int i = 0; ok && i < MARKLEN; i++)
+            ok = (m[i] == MARK[i]);
         check(ok, "mmap of a file returns the file, not zeros");
         close(fd);
     } else {