Adding new tinrc variables:

. Think of a name for your variable, we'll call it NAME
. Add it to tincfg.tbl, the ordering of this file decides the order of the
  Option 'M'enu. Use an appropriate type.
. Add an entry for it to the tin.1 manpage under the TINRC variables section
. Add NAME with the correct type to the master declaration in tinrc.h
. Add an initial value for NAME in the correct position in t_config tinrc={};
  in init.c
. In config.c, add the following:

	In read_config_file(), a match_*() function to read NAME into the
	internal tinrc table, which may not necessarily be the same type.

	In write_config_file(), code to write out NAME in some ASCII
	friendly format.

	In change_config_file(), you may need an OPT_NAME: switch to do any
	special post-processing needed when NAME is changed on the Option
	'M'enu.

. Add the following definition to extern.h
	struct opttxt txt_NAME

. Fill the above structure out in lang.c
	help text	- shown as help when asked for in Option 'M'enu
	opt text	- the text shown on the left side in the Option 'M'enu
	tinrc text	- the comment text that precedes the variable when
	              written to the tinrc file. This is optional.
