Withdraw the UNC support from splitdrive(). Instead, a new function

Withdraw the UNC support from splitdrive().  Instead, a new function
splitunc() parses UNC paths.  The contributor of the UNC parsing in
splitdrive() doesn't like it, but I haven't heard a good reason to
keep it, and it causes some problems.  (I think there's a
philosophical problem -- to me, the split*() functions are purely
syntactical, and the fact that \\foo is not a valid path doesn't mean
that it shouldn't be considered an absolute path.)

Also (quite separately, but strangely related to the philosophical
issue above) fix abspath() so that if win32api exists, it doesn't fail
when the path doesn't actually exist -- if GetFullPathName() fails,
fall back on the old strategy (join with getcwd() if neccessary, and
then use normpath()).