Arrange to strip libpq.so of symbols that aren't officially supposed to

be exported on Linux and Darwin.  We already did this on Windows but
that's not enough, as evidenced by the fact that libecpg had an unexpected
dependency on one such symbol.  We should try to do it on more platforms.
Fix ecpg's oversight, and bump libpq's major .so version number to reflect
the unwanted but nonetheless real ABI break.