Merge release-4-6 (commit 'Ic142a690')

Merge release-4-6 (commit 'Ic142a690')

This merges changes more than resolving a conflict.
The reason is that Ic142a690 moved the files to a location
not suitable for master.

molfile_plugin.h, vmddlopen.h, vmdio.h, and vmdplugin.h
were in
- "include" before Ic142a690
- "src/gmxlib" after Ic142a690
- "src/gromacs/legacyheaders" in master

Moved all but vmdio.h to "src/external/vmd_molfile"
Moved vmdio.h to "src/gromacs/gmxlib"

Moved vmddlopen.c (in src/gmxlib or src/gromacs/gmxlib) also to
"src/external/vmd_molfile" (was not moved by Ic142a690).

Removed "#ifdef GMX_USE_PLUGINS" from vmddlopen.c
This makes the files in src/external/vmd_molfile identical to
VMD 1.8.6 besides the Copyright notice.

Instead of the #ifdef, only compile vmdio.c and vmddlopen.c if
GMX_USE_PLUGINS is set (in src/gromacs/gmxlib/CMakeLists.txt)

Trivial conflict in:
	src/gromacs/mdlib/pull_rotation.c

Change-Id: If9bda639e4f680c09d785b046fc0382c16688beb