lwpCatchAwaitedSig() is not reentrant. lwpInitSigWait() attempts to
protect it by setting an appropriate signal mask, but screws up. This
could conceivably lose signals. Messed up when it got added in commit 7183516d9, v4.3.6. Fix it.
Signed-off-by: Markus Armbruster <armbru@pond.sub.org>