Reduce or eliminate build warnings

[sgillies@…]

Building the SWIG mapscript results in a frighteningly large
number of warnings.  More complete function prototyping will
be necessary to eliminate these.

[sgillies@…]

Function prototypes like

int getFoo();

result in SWIG wrapper code that warns during compilation.  I went
through various header files and changed these sort of prototypes
to

int getFoo(void);

which eliminates the warnings.  Also finished adding prototypes 
to map.h for all MapServer library functions used in mapscript.i
and pyextend.i.

Two warnings remain, associated with the lineObj and errorObj
class constructors.  They have no args, and there is some
compiler confusion, this is a SWIG issue, not something we can
fix in MapServer.