build,meson: Make feature options boolean

Different options are set as `feature`, also every possible plugin,
but this may cause confusion.

All options set as `feature` but plugins have been made `boolean` to
avoid further confusions.

`help2man` is also installed in the CI image because is required to
build different libraries.

test/meson.build

@@ -8,7 +8,7 @@ test_units = {
   'mmtty': libport_dep,
 }
 
-if gudev_dep.found()
+if enable_udev
   test_units += {'lsudev': gudev_dep}
 endif