autoconf: change AX_ARG_{DISABLE/ENABLE}_AND_EXPORT to make more sense

autoconf: change AX_ARG_{DISABLE/ENABLE}_AND_EXPORT to make more sense

Change disable/enable feature macros to have a more significative name
of what they actually do, to avoid confusions.

New macros have the following names:

AX_ARG_DEFAULT_ENABLE: feature is enabled by default, provides the
--disable-{feature} option to disable it.

AX_ARG_DEFAULT_DISABLE: feature is disabled by default, provides the
--enable-{feature] option to enable it.

Signed-off-by: Roger Pau Monne <roger.pau@entel.upc.edu>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Olaf Hering <olaf@aepfle.de>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>