dovecot: Improve packaging and include configs

dovecot: Improve packaging and include configs

- Add dovenull user, otherwise, dovecot doesn't start
- Build docs to have configuration files for dovecot
- Remove init script as conffile
- Move build options from Makefile to Config.in
- Install section to be more readable
- Refresh patches

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>
(cherry-picked from commit e27438f14082ae2f5fee3353033b0396175c855b)