Remove sysstuff.h includes from headers

Remove #include "sysstuff.h" from all headers, and replace it with
standard headers where necessary.  Add necessary standard headers to
source files that previously got them transitively from one of these.

Use snew() in gmx_spatial.c instead of a custom implementation.
Replace some free() with sfree().

Part of getting rid of sysstuff.h.

Change-Id: I857ba69c004aea16195e629c5bf69df32724b3fc
Remove sysstuff.h

Replace this with those standard headers that are actually necessary for
the source files.  There are likely several transitively covered header
dependencies now, but those can be resolved when they give problems.
One more header off from legacyheaders/.

Part of #1415.

Change-Id: I4cc3f0eec0f546dad929512e532d8efb11da3694