unifdefall: use unifdef -f and delete lots of arcane shell hackery
unifdef: increase max number of symbols to 16K

This is so that it can cope with the output of
	cc -E -dM xterm/main.c
unifdef: remove spurious \n in debug output
unifdef: fix #undef parser

Previously the parser state was incorrect following a #undef
which caused the following line to be ignored. Found when
testing unifdefall on xterm/main.c.
INSTALL: be clearer about dependencies up front
web: update for 2.8 release; clearer links to COPYRIGHT and INSTALL