Create a new section of pyport.h to hold all external function declarations

for systems that are missing those declarations from system include files.
Start by moving a pointy-haired ones from their previous locations to the
new section.

(The gethostname() one, for instance, breaks on several systems, because
some define it as (char *, size_t) and some as (char *, int).)

I purposely decided not to include the summary of used #defines like Tim did
in the first section of pyport.h. In my opinion, the number of #defines
likedly to be used by this section would make such an overview unwieldy. I
would suggest documenting the non-obvious ones, though.

Python/sysmodule.c

@@ -385,7 +385,6 @@ settrace() -- set the global debug tracing function\n\
 PyObject *
 _PySys_Init(void)
 {
-	extern int fclose(FILE *);
 	PyObject *m, *v, *sysdict;
 	PyObject *sysin, *sysout, *syserr;
 	char *s;