Fix documentation of swap/enums.h

Another issue picked up by the doxygen documentation checker.

Change-Id: Ieaad04e713bb045a018823d26983dea1d52b202f
Clean up smalloc.h

- Remove unused maxavail() and memavail().
- Remove unused scalloc().
- Remove srealloc(), which was only used in gmx_bar.c, and even there
  incorrectly.
- Move scoped_ptr_sfree to a separate header (this also potentially
  postpones issues that boost causes in CUDA code, since this is the
  only part that currently was pulling boost into CUDA).
- Convert all documentation to Doxygen.
- Fix some system header dependencies that were previously covered
  transitively, and fix some stylistic issues in the touched files.

Preparation for moving smalloc.h from legacyheaders/.

Change-Id: I89d6445799d818ccdb2bd93b6592eff1ccc618a7