Continuing 63f32f34, 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.