There is no version detection. If there is any vterm library, it is
used. If it is the wrong version, there will be an build error.
`libvterm.so.0.0.2` is preferred, which hopefully is compatible to
revision 740 or Ubuntu `libvterm0` version 0.1.2-2.
If no library is found, the correct one is downloaded and build.
* using `get_wch()` now and mapping most of the function keys documented
in `vterm_keycodes.h` (not num keypad)
* there are still a lot of mappings from ncurses missing (in conjunction
with modifier keys, related issue #11)
* using mutex to make sure writing to terminal client is serialized
Christian Burger