rework INSTALL and README to prepare for 2.6 release

rework INSTALL and README to prepare for 2.6 release

Update URLs in README

Rip out information in INSTALL that is already in PORTS, or is
printed by "./configure --help"

Update tun/tap driver information where outdated or incomplete.

Update build prerequisites, add new linux libraries, add git and
libtool to developer tools needed, etc.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20221128164932.14252-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25566.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>