[gssproxy] [PATCH] Implement dropping privileges

[Simo Sorce]

Implements RFE ticket #102

Allows to specify a user to drop privileges to.
When dropping privileges the /var/lib/gssproxy/clients directory and any
keytab gssproxy needs access to need to be readable by the proxy user.

For testing with /etc/krb5.keytab I suggest running:
setfacl -m u:gssproxy:r /etc/krb5.keytab

Assuming "gssproxy" is the name of an existing system user set in the
config file as:
run_as_user = gssproxy 

I did some basic testing on my client system that is set up to
impersonate nfs client users by using constrained delegation and the
host keytab. Seem to work ok in Fedora 20, but requires setenforce = 0
in my system because rpc.gssd is still broken in the selinux policy when
it tries to fork the handling child.

Simo.

-- 
Simo Sorce * Red Hat, Inc * New York