Added ability to remove id mappings in wbinfo and libwbclient.

Added ability to remove id mappings in wbinfo and libwbclient.

The idmap_tdb backend already provides an interface to remove existing id
mappings.  This commit plumbs that ability up through, winbindd, libwbclient,
and wbinfo.

Added new winbindd command:
        WINBINDD_REMOVE_MAPPING
Added new libwbclient interfaces:
        wbcRemoveUidMapping() and wbcRemoveGidMapping()
Added new wbinfo options:
        --remove-uid-mapping
        --remove-gid-mapping

Increased libwbclient version to 0.2
Increased winbind interface version to 20