Fix bugs and reorganize options in sqlmap 0.9.

Fix a bug causing psycopg to fail to build because pg_config is not found.

Replace --with-db-modules option with an option for each database driver. The
rationale is that some Python modules depend on a DBMS being installed (e.g., MySQL,
PostgreSQL), while others don't.

Add optional dependencies from MySQL and PostgreSQL. The dependency from MySQL
is needed only when --with-mysql is requested and only for the stable version of sqlmap,
which uses python-mysqldb. The HEAD version depends on pymysql, which does not
need MySQL to be built (the previous version of the formula always installed both
modules, but now only one is installed).