Clean up user password mailing

I recently forgot my password on a sandbox and went through the forgot my password UI to get it rather than the usual wiping the db. This propted me to clean up that function.

- After mailing the password, set a message and go to the login form (was print a message as the entire page).
- Provide feedback if neither field was filled in.
- Make it a full menu callback instead of going through user_page().
- Fixed some indentation.
- Changed some double quotes to single quotes.
- Made sure all if statements have {}.