Move the special FTN/QWKnet address formatting to a userdat-exported function

smtp_netmail_addr() - not currently used anywhere else, but may be someday.
load/mailutil.js's fidoaddr_to_emailaddr() has this same logic (for FTN addrs
at least and is now used by nntpservice.js).

Also, always pass a buffer to smb_faddrtoa() from the mail server since it's
multi-threaded and its unsafe to pass NULL (using a static local buffer).