commit b0328d5631088cca5f80acc8dd85b859f062ebb0
    Author: mcdevc <a@b>
    Date:   Fri Mar 6 16:28:45 2009 -0800

        Separate our internal libpq front end from the client libpq library
        upgrade libpq to the latest to pick up bug fixes and support for more
        client authentication types (GSSAPI, KRB5, etc)
        Upgrade all files dependent on libpq to handle new version.

Above is the initial commit of gp_libpq_fe, seems no good reasons still
having it.

Key things this PR do:

1, remove the gp_libpq_fe directory.
2, build libpq source codes into two versions, for frontend and backend,
check the macro FRONTEND.
3, libpq for backend still bypasses local authentication, SSL and some
environment variables, and these are the whole differences.
Signed-off-by: NAdam Lee <ali@pivotal.io>