Support forced identifiers ($foo) in directives. This makes yasm match

Support forced identifiers ($foo) in directives.  This makes yasm match
NASM behavior with regards to e.g. extern $foo.

Unfortunately this makes yasm start not matching NASM behavior with regards
to the RDF module directive.  In NASM, module $foo results in a literal $foo
reference; yasm now requires either a double $ (e.g. $$foo) or quotes (e.g.
"$foo").  The special case for section (section $foo --> literal $foo) is
still present, but keeping this for other directives is extremely difficult.

svn path=/trunk/yasm/; revision=1754