Allow X*IfEvent() to reenter libX11

The documentation for this family of functions very clearly says not to
call into xlib in your predicate function, but historically single
threaded apps could get away with it just fine, and now that we've
forced thread-safety on the world such apps will now deadlock instead.
That's not an acceptable regression even if the app is technically
broken. This has been reported with XFCE and FVWM, and Motif's
cut-and-paste code has the same bug by inspection, so this does need to
be addressed.

This change nerfs LockDisplay/UnlockDisplay while inside the critical
bit of an IfEvent function. This is still safe in the sense that the
display remains locked and no other thread should be able to change it
from under us, but the loop that scans the event queue might not be
robust against it being modified as a side effect of protocol emitted by
the predicate callback. But that's not new, non-XInitThreads'd xlib
would have the same caveat.

Closes: xorg/lib/libx11#157

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