libwsemul_vt100: Rumpified wscons vt100 emulator (wsvt25).

This library provides wscons vt100 emulation (aka TERM=wsvt25). This
is internal module, not a device, so there's no RUMP_COMPONENT. The
public interface provided by this module is wsemul_vt100_ops and it
needs the user to provide (in rump namespace) wsdisplay_emulinput and
wsdisplay_emulbell, provided in the real kernel by wsdisplay.

This subdirectory is not hooked into the build.