BUG #14117: psql and pg_dump commands should use ENV perl

The following bug has been logged on the website:

Bug reference:      14117
Logged by:          Joe Steffee
Email address:      jsteffee@bepress.com
PostgreSQL version: 9.5.2
Operating system:   Ubuntu 14.04.2
Description:

We have multiple systems which use non-system perl for various tasks,
including perl scripts which test some pg_dump commands. These break because
psql and pg_dump are hard-coded with #!/usr/bin/perl instead of
#!/usr/bin/env perl. The PERL5LIB is set up with libraries based on the perl
called with ENV, where /usr/bin/perl has libraries compiled differently.