Define format for usernames

[jwodder]

Currently, the only requirements for valid usernames in this spec are given at https://modern.ircdocs.horse/#user-message, where minimum & maximum lengths are discussed, but information on what characters are allowed in usernames is still lacking.

Obviously, in order for usernames to be sent as the first parameter of a USER command, they must be nonempty, must not contain NUL, CR, LF, or space, and must not begin with a colon.

Presumably, usernames cannot contain @, as otherwise there would be issues with separating the user component from the host component in a source prefix.

Previous specifications:

• RFC 1459, in its specification of target lists, states that usernames consist of one or more (or two or more? I'm not clear on the meaning of curly braces in the BNF used) "nonwhite" characters, which are defined as "any 8bit code except SPACE (0x20), NUL (0x0), CR (0xd), and LF (0xa)".

• RFC 2812, in its specification of target lists, states that usernames consist of one or more of any octet other than NUL, CR, LF, space, and @.