libmount: add mnt_context_get_excode()

libmount: add mnt_context_get_excode()

It's pretty complex task to make mount(8) and umount(8) return code
and generate error message. It seems better to do that in the libmount
rather than force all library users to duplicate mount(8) mk_exit_code()
functions. It also means that all the messages will be translated only
once. Changes:

* all error messages are printed by warn()
* no more multi-line messages
* all messages prefixed by mount target (mountpoint)
* library provides mount(8) compatible MNT_EX_* codes

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1429531
Signed-off-by: Karel Zak <kzak@redhat.com>