Make dhcp enforcing.

Change-Id: I9355cd9bcbfeb55a7bebe580c0e5764263a5f5ab
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>

dhcp.te

 type dhcp, domain;
-permissive_or_unconfined(dhcp)
 type dhcp_exec, exec_type, file_type;
 type dhcp_data_file, file_type, data_file_type;