This adds -Wno-implicit-fallthrough for libvgz to satisfy gcc7.

The -Wno-unknown-warning-option is there to satisfy older gcc/clang that
don't recognize -Wno-implicit-fallthrough.

Fixes: #2668

This eliminates read-add-zero-write on counters outside this group.

in VSC_C_main and protect them with the ban-mtx.

Conflicts:
	lib/libvcc/generate.py

reports stuff from it for debugging.

size_t is 64 bit.

Fixes: #2780

Fixes: #2777

Conflicts:
	doc/sphinx/whats-new/upgrading-6.1.rst

Text mostly taken from ecdebdc7

We copy vmods using unique filenames for ages

Ref: 3cf58766

Original text from 2014: 920541a8

Conflicts:
	doc/sphinx/whats-new/upgrading-6.1.rst

Not ready to tackle the Python2/Python3 monster a week before release.

Helps: #2764

Fixes:	#2773

The fact that the previous object was IMS/INM eligable does not make
one created from it eligable as well.

Also test for the miss + hfm case with IMS/INM

Fixes #2763