Initial commit: Schunix as it stands before the SchOS rename

An x86-64 operating system built from xv6, now largely rewritten: SMP,
a copy-on-write root filesystem (SchFS), PCI/USB/virtio drivers, a TTF
rasteriser, and a Linux-numbered syscall ABI.

No history before this point -- the tree was never under version control.
This commit exists so the rename that follows has something to undo to.

tools/padstage2.pl

@@ -0,0 +1,24 @@
+#!/usr/bin/perl
+# Zero-pads the stage2 bootloader binary out to a fixed size (STAGE2_SECTORS
+# * 512 bytes), so the kernel's on-disk starting sector is predictable
+# regardless of stage2's actual compiled size. Errors out if stage2 has
+# grown past that budget instead of silently truncating it.
+
+my $size = $ARGV[1] || die "usage: padstage2.pl <file> <size-in-bytes>\n";
+
+open(F, $ARGV[0]) || die "open $ARGV[0]: $!";
+my $n = sysread(F, my $buf, $size + 1);
+close(F);
+
+if ($n > $size) {
+  print STDERR "stage2 too large: $n bytes (max $size)\n";
+  exit 1;
+}
+
+print STDERR "stage2 is $n bytes (max $size)\n";
+
+$buf .= "\0" x ($size - $n);
+
+open(F, ">$ARGV[0]") || die "open >$ARGV[0]: $!";
+print F $buf;
+close(F);