Change main header file name (or provide alternative)

[andrew.bell.ia@…]

map.h was used to store C++ library map data structure definitions before the
standard became formal and still exists on many systems.  This can cause a
conflict which searching for the mapserver map.h file.  Suggest renaming map.h,
or providing an alternative file (mapserver.h, perhaps) which would simply
include a well-specified map.h to avoid naming conflicts:

#ifndef MAPSERVER_H
#define MAPSERVER_H

#include "./map.h"

#endif