Move gridmap parsing code into XrdOucGMap

This patch creates a new mapping service class, XrdOucGMap, out of code previously located
under XrdSecgsi. The main reason being the need to use this functionality elsewhere, namely
XrdHttp.
The patch also contains the required modifications in XrdSecProtocolgsi to use XrdOucGMap as
Grid map service; in particulare, LoadGMAP is removed and QueryGMAP modified accordingly.