multipath: Allow whitespace before or after exit and quit command in multipathd

This problem is reported by IBM internal test team.

In multipathd interactive mode, commands allow preceding and appending
whitespace while quit and exit do not. If we have whitespace before or
after quit|exit command, multipathd won't recognize exit and quit
command and exit the interactive mode.

This patch provide support whitespace in the quit and exit command.

References: bnc#846575

Signed-off-by: Zheng Huai Cheng <zhenghch@linux.vnet.ibm.com>
Acked-by: Hannes Reinecke <hare@suse.de>