Strip exec* permissions from unconfined domains.

This ensures that only domains that are explicitly allowed executable
memory permissions are granted them.

Unconfined domains retain full write + execute access to all file
types.  A further change could possibly restrict execute access to
a subset of file types, e.g. system_file + exec_type.

Change-Id: I842f5a2ac5921cc2bd0ab23a091eb808fdd89565
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>