FST: Introduce a new config option fstofs.authdir to replace hardcoded /var/eos/auth/ usages and setup the given directory at FST start up in XrdOFS::Configure.

A new config option has been introduced in the FST config file: fstofs.authdir
The option has a default value to /var/eos/auth/
This value is mandatory because FST file transfers (e.g.: replication, distributed draining) assume this folder exists, otherwise they will fail.
On el7, this folder is set up for the FST by systemd start up.
On el6, this folder must be set up manually.