preproc: fix parsing of single-line macro arguments, cleanups

preproc: fix parsing of single-line macro arguments, cleanups

The single-line macro argument parsing was completely broken as a
comma would not be recognized as an argument separator.

In the process of fixing this, make a fair bit of code cleanups.

Note: reverse tokens for smacro->expansion doesn't actually make any
sense anymore, might reconsider that.

This checkin also removes the distinction between "magic" and plain
smacros; the only difference is which specific expand method is being
invoked.

Finally, extend the allocating-string functions such that *all* the
allocating string functions support querying the length of the string
a posteori.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>