Use 'xrd.' to scope sec url params
Pass wished sec protocol list via Url param

The ordered security protocol list to be tried by the client is controled
by the env variable XrdSecPROTOCOL. For thread-safe operations, this patch
adds the possibility to pass the list via the parameter url xrd.wantprot, i.e.

    protocol://user:password\@host:port/path?xrd.wantprot=krb5,gsi,pwd
Pass credentials path via Url param

The path to the credential cache file is controlled by the env KRB5CCNAME.
This patch adds the possibility to pass it via the url parameter xrd.k5ccname,
i.e.

    protocol://user:password\@host:port/path?xrd.k5ccname=/tmp/kcc_mine
Pass proxy path via Url param

The path to the proxy file is controlled by the env X509_USER_PROXY or XrdSecGSIUSERPROXY.
This patch adds the possibility to pass it via the url parameter xrd.gsiusrpxy,
i.e.

    protocol://user:password\@host:port/path?xrd.gsiusrpxy=/tmp/u_mine
gsi: support passing cert,key paths via URLs

Extend the possibility to pass paths via option URLs to the user certificate and
key files. The new options are called 'xrd.gsiusrcrt' (for the certificate) and
'xrd.gsiusrkey' (for the key).
Fix issue with debug statement
Pass options when creating the auth protocol
Make sure all original extensions are copied to proxy