Added ./configure flag to disable user-defined scripts.

Added ./configure --disable-scripts build flag to ignore the
"script-security" directive in configs.  This can be used to
prevent configs from raising the script-security level to
allow user-defined script execution.

At the C level, script support is enabled by:

  /* Enable user-defined scripting */
  #define ENABLE_SCRIPTS 1

or disabled by ensuring that ENABLE_SCRIPTS is undefined.

If ENABLE_SCRIPTS is undefined, OpenVPN will log "[NOSCRIPTS]"
in the initial version info line.

This patch assumes that script_security defaults to
SSEC_BUILT_IN or less.

Signed-off-by: James Yonan <james@openvpn.net>