Try again to enable the idle call.

DF is undefined on entry, so make sure we clear it at all entry points
where it could possibly matter.

- save and restore all the DOS vectors around COMBOOT invocations.
- handle INT 28h and INT 29h, and IRET on INT 2Dh, 2Fh, 33h.

If we're doing an unofficial build from git, put the git id and a
dirty flag in the build string, instead of a timestamp.  This is a lot
more useful.

Allow the initrd to be specified on a separate line from command-line
options (append).  This apparently can help certain tools.

At least one laptop exists which calls INT 2Fh from inside the BIOS!
So just point INT 2Fh to IRET, and hope that's good enough.

Clean up the BSUBDIR/ISUBDIR separation further.  Build _bin.c files,
which are an intermediate stage toward building the installers, in the
libinstaller directory, since that directory is used by all the
installers anyway.  That also lets us get bin2c.pl out of the root.

- "make clean" should not clean up things that "make installer" cannot
  regenerate.
- "make installer" should not descend into subdirectories which are
  target only, e.g. com32 and sample.

The Linux platform stuff in com32 is old and bitrotted, and was only
for debugging in the first place.  Just ignore it.

The comapi functions get very unhappy if bp is ever clobbered, since
it uses bp as a frame pointer.