The following features must be added to make nail a mailx implementation as
defined by the Single Unix Specification, v2:

* -F option
* LC_TIME environment
* LC_MESSAGES environment
* Return a non-zero exit status if SIGINT was received twice and ignore not set
* Import from environ after processing options: DEAD,EDITOR,MBOX,LISTER,
  PAGER,SHELL,VISUAL
* Otherwise, environ does not affect internal mail variables? On Solaris,
  they do not, but our functionality seems useful.

* allnet variable
* bang variable
* add debug variable to man page
* flipr variable
* onehop variable
* outfolder variable
* prompt variable
* prompt defaults to "?"
* sendwait variable
* showto variable
* sign variable
* Sign variable

* Copy command
* followup command
* Followup command
* Save command

* ~a command
* ~A command
