for gcc >= 2.5 and no-ops for gcc >= 2.6. Converted to use __dead2
or __pure2 where it wasn't already done, except in math.h where use
of __pure was mostly wrong.
mount_* programs. While we're at it, collapse the four now-identical
mount programs for devfs, fdesc, kernfs, and procfs into links to
a new mount_std(8) which can mount any really generic filesystem
such as these when called with the appropriate argv[0].
Also, convert the mount programs to use sysexits.h.
