The jack_group2gid() function does a thread-safe lookup of a unix gid from a
unix group name.

The jack_promiscuous_perms() function adjusts the permissions of a shared
resource (socket, semaphore, shm segment, ...) referenced by a fd and/or a
path so it can be used by any member of the provided unix group.

By using those functions it will be possible to enable a secure promiscuous
mode. 'Secure' meaning here that one is not forced to launch every clients
with a (way) too permissive 0000 umask.