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

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.