Revert netvars to user value after demo finishes playback

See merge request STJr/SRB2!1578

Fix for #551

See merge request !1590

P_ZMovement: add a P_MobjWasRemoved check after P_CheckPosition, so we can bail out if the mobj was removed (by Lua most likely)

Actually tell you if the server is running an incompatible version

See merge request STJr/SRB2!1509

Lua userdata invalidate fix

See merge request !1584

Bugfix - Fix sporadically occurring incorrect userdata types in Lua, caused by previously loaded userdata which didn't get invalidated in previous sessions.

Invalidate userdata for line and mapthing args.
Invalidate userdata for slopes and their normal, origin and direction vectors.

Use floating-point math for polyobject planes as well.

Closes #581

See merge request !1533

(Demos do not call CL_Reset BTW.)

Take advantage of new Makefile in appveyor.yml

See merge request !1561

Also changed -Wno-error=implicit-fallthrough to
-Wno-implicit-fallthrough. For some reason Appveyor's
version of GCC is triggering these warnings despite the
comments, so just shut it up.

According to this answer--
https://stackoverflow.com/a/45952425
--Make will always prefer a unix shell, even on Windows,
if one can be found in Path. So we can't check PATH to
determine if it's a Windows shell... this is just too much
bother.

Use return value of KeyDown and KeyUp hooks

See merge request !1559

Some issues with the Makefile

See merge request STJr/SRB2!1553

If a variable is defined as in 'make CC=gcc-10', then that
definition overrides anything other definition in the
Makefile.

After a checkout from before revision, old directories
such as bin/Linux64 only remain if untracked files exist
within. This may be confusing to the user. They may even
use an outdated executable if it is one of those untracked
files.

Hooklib refactor

See merge request STJr/SRB2!1307

This is for MSYS2, which requires unix shell commands.

File function is not supported < Make 4.2.

# Conflicts:
#	src/lua_hook.h
#	src/lua_hooklib.c

Let Lua access key presses, mouse properties, and joystick properties

See merge request STJr/SRB2!1436