squish remove some mingw-specific code that is covered by gnulib

squish remove some mingw-specific code that is covered by gnulib

* libguile/socket.c (scm_init_socket): Remove mingw-specific code.

* libguile/fports.c: Remove ftruncate redefine; mingw is fine.
  (scm_i_fdes_to_port): If we have no F_GETFL, just do an fstat.  The
  right place for an F_GETFL replacement would be in gnulib.
  (fport_input_waiting): Remove an outdated comment.

* libguile/error.c (SCM_I_STRERROR, SCM_I_ERRNO): Remove, replacing uses
  with strerror and errno.

* libguile/win32-socket.c:
* libguile/win32-socket.h: Remove.  Mingw has suitable replacements.

* configure.ac:
* libguile/Makefile.am (EXTRA_libguile_@GUILE_EFFECTIVE_VERSION@_la_SOURCES):
  (noinst_HEADERS): Update for win32-socket removal.