Fix escaped backslashes in grants

* Mysql uses the underscore character to represent a single character
  wildcard.
* A grant on table `the_database`.* would match `theAdatabase`.*, so
  underscores must be escaped to avoid this match.
* The output from mysql escapes special characters (\n, \t, \0, and \\),
  but the input does not need to be escaped.
* In order for the provider to compare the tables, the output of
  mysql -NBe <query> must have \\ substituted with \.
Merge pull request #594 from skroll/escape_backslash

Fix escaped backslashes in grants