Avoid a race condition in glibc between AIO and setresuid().

Avoid a race condition in glibc between AIO and setresuid().

See this test: http://samba.org/~tridge/junkcode/aio_uid.c

The problem is that setresuid() tries to be clever about threads, and
tries to change the euid of any threads that are running. If a AIO read
or write completes while this is going on then the signal from the thread
where the IO completed is lost, as it gets -1/EPERM from rt_sigqueueinfo()

The simplest fix is to try to use setreuid() instead of setresuid(),
as setreuid() doesn't try to be clever. Unfortunately this also means
we must use become_root()/unbecome_root() in the aio code.
(This used to be commit 56c5a6f024875bb79b0104beb36f6b0ec1e1e9f9)