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/core/main.c

@@ -13,7 +13,7 @@ static void mpmain(void)  __attribute__((noreturn));
 extern char end[]; // first address after kernel loaded from ELF file
 
 // Set by stage2main.c (boot/stage2main.c, BOOT_VERBOSE_FLAG_ADDR) when the
-// user picks "Boot Schunix (verbose)" from the F8 boot menu; read once here,
+// user picks "Boot SchOS (verbose)" from the F8 boot menu; read once here,
 // before anything else, so subsystem init can log its own steps.
 int boot_verbose;
 

kernel/core/panic.c

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: Apache-2.0
 //
-// PSOGM — the Schunix crash screen.
+// PSOGM — the SchOS crash screen.
 //
 // The name is a nod to the Amiga's "Guru Meditation", but nothing else about
 // this is a nod to 1985. Three things it is built around:
@@ -439,7 +439,7 @@ paint(char *reason, addr_t *pcs, int npcs)
 
   // Left-aligned, not centred. Centred text reads as a poster; this is a
   // report, and a report is read down the left edge.
-  vga_text_at(left, y, "Schunix ran into a problem and needs to restart.",
+  vga_text_at(left, y, "SchOS ran into a problem and needs to restart.",
               PSOGM_FG);
   y += ch + ch / 2;
   vga_text_at(left, y, "Nothing on your disks was touched by this.",
@@ -492,7 +492,7 @@ report_serial(char *reason, addr_t *pcs, int npcs)
 {
   cprintf("\n");
   cprintf("  :(\n\n");
-  cprintf("  Schunix ran into a problem and needs to restart.\n");
+  cprintf("  SchOS ran into a problem and needs to restart.\n");
   cprintf("  PSOGM -- Purple Screen of Guru Meditation\n\n");
 
   for (int i = 0; i < nfields; i++) {

kernel/core/time.c

@@ -3,7 +3,7 @@
 // The kernel has always known how long it had been running -- `ticks`, bumped
 // 100 times a second by the timer interrupt -- and has never known what time it
 // was. sys_gettimeofday() returned `ticks / 100` as though it were seconds
-// since the epoch, so every Schunix machine believed it was the 1st of January
+// since the epoch, so every SchOS machine believed it was the 1st of January
 // 1970 and that the date advanced at the rate of its own uptime. `date` printed
 // a number that barely moved, which is what it looks like when a clock is
 // counting from the wrong place rather than not counting at all.