nano: enable line numbers (from cmdline)

nano: enable line numbers (from cmdline)

Enable showing line numbers, which makes it easier to find
the correct place in long files. Due to the "tiny" build,
the functionality is only available from cmdline with "-l"
option.
For example:  nano -l /etc/banner

Key shortcut M-n is disabled due to "tiny".
See https://savannah.gnu.org/bugs/?61441 for explanation.

Also
* remove without-slang config option, deprecated in Dec 2020
* adopt AUTORELEASE

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>