Revert "Renamed some global function"

This reverts commit 69728b725778a2511ca4570b66ea816dba07aca2.
molfile plugins should now be only in their PLMD::molfile namespace
(no C linkage).
voodoo attempt to remove extern from stuff which is not extern
Merge pull request #111 from tonigi/fix-110-tg

No "extern C" in molfile plugins