package/proftpd: add mod_cap option

Add an option to enable or disable mod_cap and select libcap accordingly
instead of using bundled libcap which raise a build failure with headers
< 4.3 due to PR_CAP_AMBIENT and will be removed in version 1.3.7:
https://github.com/proftpd/proftpd/commit/8c845703fcf2c7978614784126bd074ffc4477f9

Fixes:
 - http://autobuild.buildroot.org/results/4d680d8204bdf1f3deec2c3eeb9a2d9e6eabe4d5



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>