MSVS: Add {shared_library,loadable_module}_extension.
MSVS: Add {shared_library,loadable_module}_extension.

These are used by native target's code to determine file extension if
not overriden.  Because of this omission, DLLs with custom basename (but
not extension) had empty extension in vs201x toolsets.

vslavik/bakefile,vslavik/bakefile,vslavik/bakefile