xrootd: new switch -L for protocol library path

The xrootd executable allows to load an alternative default protocol whose name
is defined by the switch -P. The path to the library with the implementation of
the alternative protocol had, so far, to be specified in the configuration file.
This patch adds the possibility to specify also the library path on the command
line using the switch -L. So, protocol 'myprot' implemeted in /path/to/libmyprot.so
can now be also started with

       $ xrootd -P myprot -L /path/to/libmyprot.so
[XrdCl] Make sure to use XrdSysPlugin (versioning) when loading security plugins