MINOR: fd: Implement fd_takeover().

MINOR: fd: Implement fd_takeover().

Implement a new function, fd_takeover(), that lets you become the thread
responsible for the fd. On architectures that do not have a double-width CAS,
use a global rwlock.
fd_set_running() was also changed to be able to compete with fd_takeover(),
either using a dooble-width CAS on both running_mask and thread_mask, or
by claiming a reader on the global rwlock. This extra operation should not
have any measurable impact on modern architectures where threading is
relevant.