MFH: r429501

Fix handling of the GETENT plist substitution, add one for ypcat.

GETENT is not a port option, WITH_GETENT is defined if /usr/bin/getent is
present on the system, and the handling was broken in r319851 ("Convert to new
options framework").

While here, add a check for /usr/bin/ypcat since it is not present when world
is built with WITHOUT_NIS.

Approved by: ports-secteam (junovitch)