blindly allow any [a-zA-Z] as a snomask char

[jesopo]

would allow users to subscribe to snotes that other servers (probably mostly services) might generate using chars the IRCd doesn't know about

[jesopo]

I wonder if, instead of this, we can make this part of a netjoin burst, so services would link on and tell the rest of the network what snote chars it plans to supply?

[jesopo]

see also atheme/atheme#810