Add environment variable for GSS mech config

Add environment variable for GSS mech config

If the environment variable GSS_MECH_CONFIG is set (and the process is
not privileged), read it instead of /etc/gss/mech or files within
/etc/gss/mech.d.

Set GSS_MECH_CONFIG in test frameworks so that system configuration
does not interfere with tests.

Fix documentation to indicate that the default mech config file is in
sysconfdir, not necessarily /etc.

ticket: 8833 (new)