This change splits mysql_grant based on the type of grants issued.

* mysql_grant_global -> GRANT <priv> ON *.*
* mysql_grant_db     -> GRANT <priv> ON <database>.*
* mysql_grant_proxy  -> GRANT PROXY ...

This does introduce a bit of code duplication, however it allows further
refactoring and also isolates the grant modifying logic away from other
details such as table opening, locking, binlogging.

Another side effect is that the mysql.db table is only locked for a
database level grant. Before it was locked even if there were only
global level grants issued.