Allow username to take no argument to erase value
Once a username has been set, it could not be removed via the UI. Now, if no
argument is passed to username, the value stored is replaced with NULL.
# All parameters are described in pgsql_table(5) / PGSQL PARAMETERS
#
# The hosts that Postfix will try to connect to and query from.
hosts = localhost
# The user name and password to log into the pgsql server.
user = postfix
password = some_password
# The database name on the servers.
dbname = mailsys
# The SQL query template used to search the database
query = SELECT uid FROM postfix_virtual_uid_map('%u', '%d')