Move execve/run_script helper functions to run_command.c

Move execve/run_script helper functions to run_command.c

To avoid having to include misc.c - which is a dependency mess - in the
tls-crypt unit tests, move the command execution helper functions to a new
run_command.c module.

While at it, abstract away the script_security global variable.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20180704175404.22371-2-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17212.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>