Document common uses of 'echo' directive, re-enable logging for 'echo'.

Document common uses of 'echo' directive, re-enable logging for 'echo'.

The 'echo' command can be used to signal information to an OpenVPN
GUI driving the openvpn core via management interface.  Which commands
exists and their syntax has so far been mostly undocumented.

Condense the long and good discussion between Selva Nair and
Jonathan K. Bullard into doc/gui-notes.txt (initial draft from
Jonathan, comments from Selva and Arne), with a pointer added
to doc/management-notes.txt.

See:

https://sourceforge.net/p/openvpn/mailman/openvpn-users/thread/CAEsd45T%2Bd
6FUJ9Po0KHwtHjfuL9Q2D-poG8yFtY45Qyh%2BtHjkg%40mail.gmail.com/#msg36136236

and

https://sourceforge.net/p/openvpn/mailman/openvpn-devel/thread/CAKuzo_jPThh
vXTJAtzhqVUVOLPW1VGu6h2jQhVsHicY8P2WRqA%40mail.gmail.com/#msg36141193

for the details.

Re-enable logging of 'echo' statements, but only for the particular
class of messages starting with 'echo msg...'.

v2:
  incorporate feedback from Selva Nair, correct >ECHO examples

v3:
  add "msg*" support status for Windows GUI (11.22.0) and Android (Planned)

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <20210118162850.24214-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21443.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit ef2405a6bf5e8159d2e51e45107bc280fd6d0bd3)