Most of the PDB stuff should be moved to bfd. This commit sets that up
and adds a reference to it from coffread.c.
The PDB path resolving will still be in gdb.
Because we weren't fully setting up the psymtab, we ended up
dereferencing invalid/null pointers when its functions were called. For
now, we just deal with 'no symbols found' errors, because it's better
than crashing.
Fixes main issue in #2
