gh-108765: Remove old prototypes from pyport.h (#108782)

Move prototypes of gethostname(), _getpty() and struct termios from
pyport.h to the C code using them: posixmodule.c, socketmodule.c and
termios.c.

Replace "#ifdef SOLARIS" with "#ifdef __sun".

Modules/posixmodule.c

@@ -58,6 +58,13 @@
 #include <stdio.h>                // ctermid()
 #include <stdlib.h>               // system()
 
+// SGI apparently needs this forward declaration
+#ifdef HAVE__GETPTY
+#  include <sys/types.h>          // mode_t
+   extern char * _getpty(int *, int, mode_t, int);
+#endif
+
+
 /*
  * A number of APIs are available on macOS from a certain macOS version.
  * To support building with a new SDK while deploying to older versions