dhclient script in rc.d doesn't use ${dhcp_program} (conf/53007)

Hi folks-

I'm happily using 5.1 and it's terrific.  Keep up the great work.

I just submitted a PR for a bug I noticed in the dhclient script.  Namely, 
it ignores the setting of dhcp_program from rc.conf.  A one-line fix did 
the trick for me, although there may be ramifications I'm not aware of.

The PR is conf/53007.

JN