Add option to force uid/gid

[joshbowyer]

There should be a configurable option in umtprd.conf to force uid/gid for files written to mount loops, such as samba does with the force user and force group directives. As in, if I specify force user = 1000 for the /home loop, then any files written via to /home via umtp should have 1000:1000 ownership (which would translate to a user/group on the host system)

[jfdelnero]

We will consider this feature and study how this can be implemented.

[jfdelnero]

@joshbowyer The UID / GID can now be forced for each storage unit.
currently on the master branch. Configuration file updated showing the "default_uid/default_gid" and "uid=/gid=" usage.