Fix SQL wildcard usage in util_uninstall()

Underscore is a SQL wildcard that actually represents any single character, so util_uninstall() may delete all variables starting with 'util'.

This line in util_uninstall():

  db_query("DELETE FROM {variable} WHERE name LIKE '%s%%'", 'util_');

Would have to be replace into something like this:

  db_query("DELETE FROM {variable} WHERE name LIKE '%s%%'", 'util\_');

Here, underscore is escaped with a backslash so it is treated by the SQL server as a literal character, not a wildcard character.