meson: Apply meson guidelines

Following the meson porting guidelines[0], this patch renames the
`with-ca-certificates` build options. The following rules has been
applied:

- Remove the with prefix from string options.
- The character separator from multi-word options has been changed
  to underscore.

https://bugzilla.gnome.org/show_bug.cgi?id=793311

[0] https://wiki.gnome.org/Initiatives/GnomeGoals/MesonPorting