For the first few versions we do not want to intercept all gssapi callers by default, as there will be bugs and we want to be able to manually control which applications actually go through the gssproxy one by one easily.

The simplest way to do this is by return NULL from gss_mech_interposer by default unless a specific environment variable is set.

At the moment we already have this option but it is reversed (if no env var is available the plugin operates) and has a name not suitable for external use (_GSSPROXY_LOOPS).

_GSSPROXY_LOOPS should probably changed to GSS_USE_PROXY = YES|NO
And the code in the gssproxy server that sets this to avoid looping on itself changed accordingly.

As for what default behavior to use we have 2 options:
1. Use a ./configure option that sets what should be the default if no env var is defined.
2. Or a plugin option read from the gss.conf file that defines the default beahvior.

The second option is interesting because it would allow us to experiment w/o having to rebuild the code, so I would probably lean more on that side.