Improve handling of password reuse in src/bin/scripts programs.

Improve handling of password reuse in src/bin/scripts programs.

This reverts most of commit 83dec5a71 in favor of having connectDatabase()
store the possibly-reusable password in a static variable, similar to the
coding we've had for a long time in pg_dump's version of that function.
To avoid possible problems with unwanted password reuse, make callers
specify whether it's reasonable to attempt to re-use the password.
This is a wash for cases where re-use isn't needed, but it is far simpler
for callers that do want that.  Functionally there should be no difference.

Even though we're past RC1, it seems like a good idea to back-patch this
into 9.5, like the prior commit.  Otherwise, if there are any third-party
users of connectDatabase(), they'll have to deal with an API change in
9.5 and then another one in 9.6.

Michael Paquier