m_mysql Fix escaping strings longer than MAXBUF/2

Quotes from the documentation:
"You must allocate the to buffer to be at least length*2+1 bytes long. (In the worst case, each character may need to be encoded as using two bytes, and you need room for the terminating null byte.)"

"The return value is the length of the encoded string, not including the terminating null character."

http://dev.mysql.com/doc/refman/5.6/en/mysql-real-escape-string.html
cmd_who Hide +i users when listing users on a server and hidewhois is off

Fixes #547 reported by @RawrDragon