Harmonize and improve functions to parse epsg codes in OGC filter encoding

There are duplicate functions FTLParseEpsgString() and FLTParseEpsgString() in mapogcfiltercommon.c and mapogcfilter.c returning a projectionObj for a given epsg urn/uri/code. This patch removes FTLParseEpsgString() in favour of FLTParseEpsgString() and also references it in FLTApplySimpleSQLFilter() where independent code was used before.

Additionally, FLTParseEpsgString is more clearly and supports the OGC URN expression for epsg codes, too.