Move env helper functions into their own module/file

Move env helper functions into their own module/file

To avoid a dependency on misc.c - which is a dependency mess - in the
tls-crypt unit tests, split the env_set functionality out into it's own
file.

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1512734870-17133-8-git-send-email-steffan.karger@fox-it.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16066.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>