Add -d option to pg_dumpall, for specifying a connection string.

Like with pg_basebackup and pg_receivexlog, it's a bit strange to call the
option -d/--dbname, when in fact you cannot pass a database name in it.

Original patch by Amit Kapila, heavily modified by me.
Only quote libpq connection string values that need quoting.

There's no harm in excessive quoting per se, but it makes the strings nicer
to read. The values can get quite unwieldy, when they're first quoted within
within single-quotes when included in the connection string, and then all
the single-quotes are escaped when the connection string is passed as a
shell argument.