client: Tie up a few lose readline ends

client: Tie up a few lose readline ends

Document readline in more detail in man/empire.6.

Make @history_file local to main().

main() silently truncates the home directory name to 1000 characters
when constructing the history file name; mark FIXME.

Set @rl_already_prompted just once.

Write history file on unsuccessful exit, too.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>