lwp: Fix handling of sigismember() failure

lwp: Fix handling of sigismember() failure

sigismember() fails when passed an invalid or unsupported signal
number.  lwpInitSigWait() and lwpGetSig() treat sigismember() failure
like "is a member".  lwpInitSigWait() will then sigaction()
unsuccessfully.  Harmless.  lwpGetSig() returns the bad signal number
when it's greater than any caught signal's number.  The bad signal
number then gets returned to main(), which shuts down the server.

Fix by treating failure like "is not a member".

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>