I don't know Centos. I don't use Postgresql from packages, because it isn't perfect never (its my experience...). Always I compile Postgresql-Postgis from source and I'm happy. In my opinion when the maintainer made the postgis package doesn't put the shp2pgsql / pgsql2shp program to the package. This problem is independent that you can to run postgis_full_version SQL function and there isn't shp2pgsql, that is a "additional" binary software, isn't SQL function. I guess you must try to complie & install & use postgresql-postgis from source... or must wait that Centos Team improving the mistake. Or: compile postgresql-postgis from source and copy the shp2pgsql / pgsql2shp to the /usr/local/bin (for example).