a bunch of postgresql related bugs

[rstml]

just seen pgsql suport and decided to test, however
looks like no one use it:

- 'ip' column missing in 'session' table
- postgres do not support LIMIT #,# syntax, use LIMIT
and OFFSET clauses
- there's no NOW() function, use now() instead
- change ORDER BY `default` to ORDER BY "default"

And sort of feature request:
- add prefix to table names. it's useful to have
"rc_sessions" table instead of "sessions" when you've
limited number of databases.
- add dynamically configurable sequence names

[nobody]

Logged In: NO 

Postgres does not support UNIX_TIMESTAMP()

use:

EXTRACT(EPOCH FROM current)

[nobody]

Logged In: NO 

another minor postgresql mistake in INSTALL (might be freebsd specific) :

roundcubemail =# ALTER USER roundcube WITH PASSWORD 
'the_new_password';   

should be changed to 

roundcubemail =# ALTER USER roundcubemail  WITH PASSWORD 
'the_new_password';   

or the user should be called roundcube everywhere and not roundcubemail