Use a template for 'unsupported management commands' error

Use a template for 'unsupported management commands' error

The message
"ERROR: The 'foo' commmand is not supported by current daemon mode"
is repeatedly used in manage.c. Move it to a function for uniformity
in messaging.

v3, v3: no change
Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20210907223126.8440-3-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22814.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit caf1b9030f28a984a0e12dd87b128b428d3683f0)