Groff should not permit ANSI escapes using \N'27'

[John Gardner]

Hi folks,

Raw escape characters (U+001B) get stripped from source-code during
formatting, but inserting one is still possible using \N'27':

\N'27'[4mI don't remember underlining this.\N'27'[0m

This has potential security implications for people using `less -R` (and
can still mess up terminal output for those who don't).

mandoc replaces this with <ESC> for -Tascii output, and with U+FEFF (�) for
-Tutf8. Its -Tlint "device" doesn't complain about it, either.