PostgreSQL support with required code modifications

I've managed to get webforms up and running on Postgres. Following is a list of changes:

• the install file didn't support PostgreSQL
• wrong use of db_next_id() caused PostreSQL errors
• double quotes (") can't be used for literal strings

db_next_id() seems to be a frequent cause of confusion, as most developers program for MySQL exclusively. The correct (ie. Postgres-compatible) format is db_next_id('{tablename}_columnname');. "columnname" should exactly match the column name the sequence is intended for.

Double quotes for literal strings seem to be not ANSI-compliant, so Postgres treats the string as identifier (table name). Switched on using single quotes instead.

Although all changes are for Postgres, i've added an update path for MySQL users to fix the sequence naming; they should run update.php after patching.