Moved PG_LWGEOM WKB I/O and SRID get/set funx

Moved PG_LWGEOM WKB I/O and SRID get/set funx
from lwgeom_api.c to lwgeom_pg.c.
Made lwgeom_from_ewkb directly invoke grammar parser rather then invoke
the PG_LWGEOM-specific function.
Cleaned up signedness-related and comments-related warnings for the files
being committed (more to do on other files)

git-svn-id: http://svn.osgeo.org/postgis/trunk@2243 b70326c6-7e19-0410-871a-916f4a2858ee