semaphore: add available_units()
semaphore: add available_units()

With the introduction of semaphore::consume() the internal counter can
go below zero. semaphore::consume() hides this fact and returns 0 in
such cases but it may be desirable to get the real value.

Signed-off-by: Paweł Dziepak <e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>
Message-Id: <20161201172437.1803-1-e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>

dreamsxin/seastar,avikivity/seastar,cloudius-systems/seastar,cloudius-systems/seastar,syuu1228/seastar,dreamsxin/seastar,scylladb/seastar,avikivity/seastar,avikivity/seastar,dreamsxin/seastar,syuu1228/seastar,scylladb/seastar,cloudius-systems/seastar,syuu1228/seastar,scylladb/seastar