envconfig: Add missing MachineInfo is_<os> methods
envconfig: Add missing MachineInfo is_<os> methods

mesonlib defines is_sunos, is_dragonflybsd, and is_freebsd, which
MachineInfo does not. This should be corrected.

becm/meson,pexip/meson,QuLogic/meson,pexip/meson,jpakkane/meson,QuLogic/meson,becm/meson,jpakkane/meson,QuLogic/meson,jpakkane/meson,pexip/meson,jpakkane/meson,mesonbuild/meson,QuLogic/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,becm/meson,jpakkane/meson,becm/meson,QuLogic/meson,pexip/meson,pexip/meson,becm/meson,mesonbuild/meson,QuLogic/meson,mesonbuild/meson,pexip/meson,jpakkane/meson,jpakkane/meson,becm/meson,becm/meson,jpakkane/meson,mesonbuild/meson,pexip/meson,pexip/meson,QuLogic/meson,becm/meson,becm/meson,QuLogic/meson,pexip/meson,QuLogic/meson,jpakkane/meson,mesonbuild/meson,becm/meson,pexip/meson,mesonbuild/meson