[HowTo] Replace courier with dovecot

[aseques]

Ouch, I'm installing ispcp in vz containers, so there's no default user created, the first user is created with uid 1000, while with a normal installation the first user is created during the install and so the vmail user will be 1001.
To make the install more standard it would be cool that the installer itself used the uid 1001 (if available).
PS: I'll try to keep this in mind for my next install ...