package/socat: drop optional readline dependency

socat is licensed under GPL-2.0, which is not license compatible with
readline (GPL-3.0+), so drop the optional dependency and add a comment
explaining why.

This also matches how socat is packaged in Debian, where the man page has
the following snippet added:

READLINE

  Uses GNU readline and history on stdio to allow editing and reusing input lines (example).
  Due to licensing restrictions the readline feature is disabled in Debian.  See BUGS.
  You can use STDIO instead.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>