irc: strip mIRC formatting, drop 004/005 noise, fix a RAM-bank-swap landmine

Against the real irc.sl0p.foo (via a TLS-stripping socat proxy) the
client's screen filled with garbage after connect. Two causes:

1. Color/format codes. The MOTD is a mIRC-color ASCII-art logo. We
   dropped the \x03 control byte (term ignores <32) but left its numeric
   *arguments*, so the logo rendered as digit soup ('09> stream the
   work', '030903 030903 ...'). strip_fmt() now removes \x03 color (+its
   fg[,bg] digits), \x04 hex color, and the \x02/\x0f/\x11/\x16/\x1d/
   \x1e/\x1f toggles, applied to the trailing text of every line (leaving
   \x01 for CTCP). Standalone digits like '3 apples' are untouched.

2. The real bug: handle() defaulted pfx/txt to a read-only "" literal,
   and both bang(pfx) and strip_fmt(txt) write a NUL terminator into it.
   That literal lives in ROM (000-FFF); on MBC5 a write there is a
   RAM-bank-select, silently swapping the  cart-RAM bank - where
   every static lives - out from under the program. One prefix-less or
   empty-trailing server line (real ircds send them: NOTICE AUTH, ERROR,
   registration PING) and all state turns to garbage. Fixed with a
   writable 1-byte 'empty' default (zeroed in main; gbos doesn't clear
   BSS).

Also suppress 004 (MYINFO) and 005 (ISUPPORT) - pure noise on 40 cols,
and InspIRCd splits ISUPPORT across several lines.

Verified end to end against the live server: full InspIRCd MOTD (logo,
LUSERS, links) renders clean and stable; host unit tests cover color/
bold/CTCP-action stripping and prefix-less NOTICE/ERROR/PING.

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