Modernize write_inpfile

Modernize write_inpfile

Used std::string, gmx::TextOutputFile and gmx::TextWriter to simplify
and permit testing.

To do so, modified the interface of various helper functionality so
that it can use a TextWriter that the caller could build with
something other than a FILE stream if they wanted to. This would also
be more convenient for testing them, if we ever do that.

Moved nice_header to utility/niceheader.h, renamed it
niceHeader, and permitted its caller to specify the comment character.

Moved string-handling behaviour out of cuda_version_information,
while ensuring no C++11 functionality is needed.

Updated documentation of intended scope of sysinfo.h, and removed c++
guard.

Change-Id: I6ab250c982a8e9e9ee3dacb6575ccc04f17e689c