selinux: shorten the policy capability enum names

The SELinux policy capability enum names are rather long and follow
the "POLICYDB_CAPABILITY_XXX format".  While the "POLICYDB_" prefix
is helpful in tying the enums to other SELinux policy constants,
macros, etc. there is no reason why we need to spell out
"CAPABILITY" completely.  Shorten "CAPABILITY" to "CAP" in order to
make things a bit shorter and cleaner.

Moving forward, the SELinux policy capability enum names should
follow the "POLICYDB_CAP_XXX" format.

Signed-off-by: Paul Moore <paul@paul-moore.com>