Fix #223: Correctly handle "./" paths in dwarf2.

As dwarf2 has a directory table in addition to a filename table, we split
the provided pathname when generating the filename table.  This correctly
combined both "./foo" and "foo" into a single filename table entry.
However, we were only matching on pathname instead of dir+filename when
actually generating the dwarf opcodes, resulting in not finding the "./foo"
version.  Fix to check both dir and filename separately, splitting the same
way we do when generating the filename table.

svn path=/trunk/yasm/; revision=2367