socket-util: Move get_max_fds() to process.c.

socket-util: Move get_max_fds() to process.c.

get_max_fds() is used only from process.c. Move it there
along with rlim_is_finite(). Since process_start() can only
be called before any additional threads are created, we
no longer need the thread safety checks in get_max_fds().

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>