Move CLI code to common

None of it depends on D1 vs D2.

SConstruct

@@ -1637,6 +1637,9 @@ class DXXArchive(DXXCommon):
 'arch/sdl/joy.cpp',
 'arch/sdl/rbaudio.cpp',
 'arch/sdl/window.cpp',
+'main/cli.cpp',
+'main/cmd.cpp',
+'main/cvar.cpp',
 'maths/fixc.cpp',
 'maths/rand.cpp',
 'maths/tables.cpp',
@@ -1797,15 +1800,12 @@ class DXXProgram(DXXCommon):
 'main/aipath.cpp',
 'main/automap.cpp',
 'main/bm.cpp',
-'main/cli.cpp',
-'main/cmd.cpp',
 'main/cntrlcen.cpp',
 'main/collide.cpp',
 'main/config.cpp',
 'main/console.cpp',
 'main/controls.cpp',
 'main/credits.cpp',
-'main/cvar.cpp',
 'main/digiobj.cpp',
 'main/effects.cpp',
 'main/endlevel.cpp',