Fixing major bug.

While creating user directories to store messages, server truncated
path in the wrong place, resulting in incorrect directory name and thus
blocking subsequent access to messages.
Defining clear rules for usernames and passwords.

Usernames and passwords must now adhere to clear rules: usernames
characters are required to fall in the class [0-9a-z]; password ones in
[0-9A-Za-z ].
These rules are enforced in client only, server will directly copy from 'users'
file. Therefore, server administrators must check credentials while creating
said file, or clients would not be able to log in.

Previous behaviour was quite unpredictable.