Log Message:
Use bsd.pkg.install.mk infrastructure to create the `gnats' user and
some directories. In the process kill PLIST @exec -except for those related
to install-info.
While here substitute some leftover `/usr/pkg' in MESSAGE.

XXX Marking this package as USE_BUILDLINK2 will make my life easier
for forthcoming changes but as it is not really necessary for the build
it self I guess it is not a good option?