unifdef: refactor keyword and symbol matching

The generic strlcmp() is replaced by the more specific matchsym()
and findsym() now includes the effect of skipsym().

This is to prepare for case-insensitive keyword matching, as part
of foreign language support.
INSTALL: use GitHub for Windows to run Unix shell scripts

README: update FreshMeat -> FreeCode and note that GitHub
zip file downloads are broken.

Suggested-by: Lee M. Hamel <lee.m.hamel@intel.com>