Avoid race on dvdl with Verlet+OpenMP+LINCS+FE+VV

Also restructured the dH/dlambda reduction in do_lincs (used for
coordinates and not affected by the race issue) to work similar
do the do_lincsp code and properly use thread parallelization.
Fixes #1647.

Change-Id: I4eeb131018abca88b3635932491d99a779e16037