Save and restore errno around FUTEX_WAIT futex(2) operations. While there

Save and restore errno around FUTEX_WAIT futex(2) operations.  While there
remove the unused _wait() function in librthread such that we don't have to
add the save/restore sequence there.

Fixed building Python as a race with another thread unlocking a futex(2)
would make futex(2) set errno to EAGAIN which would confuse Python in
beleiving that readdir(2) failed instead of reaching the end of the
directory.

Spotted and tested by tb@
ok bluhm@