Read /etc/gss/mech if no mech.d/*.conf found

Always read /etc/gss/mech, even if globbing /etc/gss/mech.d/*.conf
doesn't work.  Doing this using GLOB_DOOFFS proved error-prone, so use
a simpler approach: factor out the per-pathname handling into a helper
function load_if_changed, call it with MECH_CONF before the glob, then
pass each glob result through the helper.

ticket: 7925