Introduce post_process_mac_perms script.

usage: post_process_mac_perms [-h] -s SEINFO -d DIR -f POLICY

Tool to help modify an existing mac_permissions.xml with additional app certs
not already found in that policy. This becomes useful when a directory
containing apps is searched and the certs from those apps are added to the
policy not already explicitly listed.

optional arguments:
  -h, --help            show this help message and exit
  -s SEINFO, --seinfo SEINFO
                        seinfo tag for each generated stanza
  -d DIR, --dir DIR     Directory to search for apks
  -f POLICY, --file POLICY
                        mac_permissions.xml policy file

Change-Id: Ifbaca3b3120874a567d3f22eb487de1aa8bda796
Signed-off-by: rpcraig <rpcraig@tycho.ncsc.mil>