Add SNMP subsystem option to man page

The keepalived(8) man page did not mention the -x option to enable the
SNMP subsystem. This patch adds the -x (and --smmp) options to the
keepalived(8) man page, as described in the keepalived help message.
Merge pull request #91 from rohara/keepalived-man-snmp

Add SNMP subsystem option to man page