Apparently you might get strange paths from GetModuleFileName(), such as
short path names or UNC filenames, so this avoids that problem. Since you have
to tapdance with linking different libraries and defining macros depending on
what Windows you plan to target, we dynamically load the API we need, which
works on all versions of Windows (on Win7, it'll load a compatibility wrapper
for the newer API location).
What a mess.
This also now does the right thing if there isn't enough space to store the
path, looping with a larger allocated buffer each try.
Fixes Bugzilla #2435.
