Improve login

Summary:
Improve login code

Notify when the pass has been recognized, but not for a channel
the user is.

Improve verify_pass code style:

• avoid useless explicit cast, as variable is already this type
• avoid assignation in if clause
• extract complex condition in has_access_for_this_channel()

Test Plan:
Create an access #anotherchan in userlist.db

Then try to login:

18:25:09 <Dereckson> login ...
18:25:09 -Ephemereal- Pass verified but access is only for #anotherchan.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D883