jail: SIGSEGV must not be forwarded to the child process

jail: SIGSEGV must not be forwarded to the child process

A segfault in ujail caused ujail to hang with no chance to abort.
Raising the debug level revealed that SIGSEGV was delivered to
the child process instead of handled directly by ujail. The
corresponding debug message was triggered infinitely again and
again:

	forwarding signal 11 to the jailed process
	forwarding signal 11 to the jailed process
	forwarding signal 11 to the jailed process
	forwarding signal 11 to the jailed process
	forwarding signal 11 to the jailed process
	forwarding signal 11 to the jailed process
	forwarding signal 11 to the jailed process
	[...]

Signed-off-by: Leonardo Mörlein <me@irrelefant.net>