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.

tools/mkiso.sh

@@ -1,15 +1,15 @@
 #!/bin/sh
-# Build a bootable Schunix ISO.
+# Build a bootable SchOS ISO.
 #
 #   tools/mkiso.sh [output.iso]
 #
 # The disc boots through El Torito's "no emulation" mode, which is the only
-# one of the three that fits how Schunix actually loads itself.
+# one of the three that fits how SchOS actually loads itself.
 #
 #   floppy emulation      caps the image at 2.88 MB; the ramdisk-root kernel
 #                         alone is bigger than that.
 #   hard disk emulation   presents the image through the BIOS's INT 13h, and
-#                         Schunix's loader does not use INT 13h -- it drives
+#                         SchOS's loader does not use INT 13h -- it drives
 #                         the IDE controller's registers directly, which
 #                         cannot reach a BIOS-emulated drive.
 #   no emulation          the BIOS loads a chosen run of sectors at 0x7C00
@@ -23,7 +23,7 @@
 #
 # STAGE2_SECTORS comes from the environment, set by the Makefile, because this
 # same number also lives in boot/bootasm.S, boot/stage2main.c and the dd offsets
-# in schunix.img's recipe. It was previously written out by hand in three
+# in schos.img's recipe. It was previously written out by hand in three
 # separate spots in this file alone, and raising it for the boot logo is what
 # turned that from a latent hazard into a broken image.
 #
@@ -34,7 +34,7 @@
 set -e
 
 BUILD=${BUILD:-build}
-OUT=${1:-schunix.iso}
+OUT=${1:-schos.iso}
 STAGE2_SECTORS=${STAGE2_SECTORS:-48}
 STAGE2_BYTES=$((STAGE2_SECTORS * 512))
 LOAD_SIZE=$((1 + STAGE2_SECTORS))
@@ -59,7 +59,7 @@ stage2_sz=$(wc -c < "$STAGE2")
 rm -rf "$STAGING"
 mkdir -p "$STAGING/boot"
 
-cat "$CDBOOT" "$STAGE2" "$KERNEL" > "$STAGING/boot/schunix.bin"
+cat "$CDBOOT" "$STAGE2" "$KERNEL" > "$STAGING/boot/schos.bin"
 
 # xorriso if it is here, mkisofs otherwise. Both accept the El Torito flags
 # below unchanged -- xorriso's -as mkisofs mode exists precisely to be
@@ -79,9 +79,9 @@ fi
 
 $MKISOFS \
     -quiet \
-    -V SCHUNIX \
-    -A "Schunix" \
-    -b boot/schunix.bin \
+    -V SCHOS \
+    -A "SchOS" \
+    -b boot/schos.bin \
     -no-emul-boot \
     -boot-load-size "$LOAD_SIZE" \
     -o "$OUT" \