Add PostgreSQL home page to --help output

Example:

initdb --help
...
Report bugs to <pgsql-bugs@lists.postgresql.org>.
PostgreSQL home page: <https://www.postgresql.org/>

I think this is useful.  You see this nowadays in other packages as 
well.  See also 
<https://www.gnu.org/prep/standards/standards.html#g_t_002d_002dhelp> 
for a reference.

Autoconf already has a way to register the package home page and 
propagate it, so I used that.  That also makes it easier to change it 
(see http: -> https:) or have third parties substitute their own contact 
information without destroying translations.

While at it, I also did the same refactoring for the bug reporting 
address (which was also recently changed, so this is a bit late, but who 
knows what the future holds).

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services