[public][ulib][dev] Add __FALLTHROUGH macro for warning suppression

GCC 7 enables -Wimplicit-fallthrough with -Wextra, so we'll get it.
Clang has it available, so start using it and clean up code accordingly.

The GCC feature will actually recognize some formulaic comments to
suppress the warning. But there's also an explicit syntax, and
C++17 added a standard one. So use that instead, via the new macro
__FALLTHROUGH in <zircon/compiler.h>.

TC-36 #comment Clean up code in preparation for more -Wimplicit-fallthrough

Change-Id: I28309c91c32db1313d52f4255836c5ea687e141f

[virtcon] Add support for hotplugging

This change adds support for hotplugging to the virtcon. If multiple
displays are connected, the virtcon will use the display which was
attached first. The virtcon's contents and scrollback are kept when
switching between displays, although long lines will be truncated if
the virtcon moves to a smaller display.

When no display is connected, the console continues to function
normally, except without rendering anything. All checks for whether or
not any gfx operations should be done are in vc-device.

Change-Id: I467b9b1792f4d43803b521ec564dfe1873b85a97

Handle empty parameter for SGR control function

An empty parameter substring represents the default value, which is
defined to be 0 (i.e. reset attributes) for SGR.

Change-Id: Ib6e818d3dd304b51878f91154003f791bad543b4

[virtcon] cleanup

- remove some unused header includes
- remove unused vcdebug.h (xprintf was not used and now virtcon
filters its own debug prints out of the debug log console so
printf is safe to use if needed)
- clean up rules.mk to better reflect shared sources, libraries
- move shared code from main.cpp to vc-input.cpp
- no longer include main.cpp in virtual-console-test
- remove global lock, no longer needed in new single threaded design

Change-Id: I5aacc0f95f817f25ce96bbfa2f74d4877e9c4a16

[gfxconsole][virtcon] move and rename gfxconsole to virtcon

It's no longer a device, it's time for it to leave the
land of devices.

Change-Id: I977c56dbacb0794e6f67daac1dd6e3f4f2632f3d