Absolute paths in BFD

[Timothy Wall]

I've noticed quite a few instances in BFD where absolute paths are
tested by expecting the first character to be '/' (grep for "== '/'" to
see how often this is done).  This test breaks under cygwin when drive
letters are used as part of a pathname.

There should probably be a bfd function for absolute path-ness, and that
called instead of explicitly doing the leading slash test.

I was about to make a small patch when I realized that this check is
going on in many more places than the bug I was tracking.

T.