Minor vec.h cleanup

- Remove unused and not defined vecinvsqrt() and vecrecip().
- Remove PR_VEC() macro and replace the few remaining uses with the
  expansion.
- Move all C++ overloads to the same place in the file.
- Add a few missing gmx_inline attributes.
- Combine all #ifdef GMX_SOFTWARE_INVSQRT blocks, and move macros
  required only for it inside the #ifdef.
- Remove tabs in comments.

Change-Id: I59b8129a813d70b4887f9e6d00e72d2b256acfe4