S9 LIB  (troffify-char char)      ==>  string
        (troffify-string string)  ==>  string

TROFFIFY-CHAR converts a char to a string that is save for
inclusion in a TROFF document. TROFFIFY-STRING does the
same for a string.

Note: the conversion "$" -> "$dollar$" is a local idiosyncrasy.

(troffify-char #\$)           ==>  "$dollar$"
(troffify-string "\"a\\b\"")  ==>  "\"a\\\\b\""
