Switch dependency order of libpgcommon and libpgport

Switch dependency order of libpgcommon and libpgport

Continuing 63f32f3416a8b4f8e057dc184e8e8eae734ccc8a, libpgcommon should
depend on libpgport, but not vice versa.  But wait_result_to_str() in
wait_error.c depends on pstrdup() in libpgcommon.  So move exec.c and
wait_error.c from libpgport to libpgcommon.  Also switch the link order
in the place that's actually used by the failing ecpg builds.

The function declarations have been left in port.h for now.  That should
perhaps be separated sometime.