.my.cnf: default to login when no user specified

When using a .my.cnf file, when there is no user variable defined,
default to the login user.

This change has the mysql_user module behavior match the behavior
of the mysql command-line client.

Also adds an example .my.cnf to the docs.
Merge pull request #2404 from lorin/mysql-mycnf-user

.my.cnf: default to login when no user specified