Move set_proc_name() to OS specific files.

Move handling to change process name to POSIX specific files
plus add a better error message to cover the case where the
feature isn't supported.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Acked-by: Juan Quintela <quintela@redhat.com>
Acked-by: Richard Henderson <rth@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>