updated faq

[Petr M. <mi...@ph...>]

I've modified faq.tex on sourceforge:
- Added questions about pm3d, mousing and hotkeys.
- Added and modified various questions related to the forthcoming
  gnuplot 4.0.
- Various small or large changes and fixes, e.g. URLs.

Please have a look too.

There are definitely some URLs that needs to prove whether they exists.

There is a (for me) "strange section" on pgm, pbm et al formats and
utilities.
Maybe it needs a rewrite?

---
Petr Mikulik