Fix problems with Windows file names that use backslashes.

Fix problems with Windows file names that use backslashes.

* libguile/load.c (scm_i_mirror_backslashes): New function.
  (scm_init_load_path): Call it to produce MS-Windows file names
  with forward slashes.
  (FILE_NAME_SEPARATOR_STRING): Define as "/" on all platforms.

* libguile/load.h (scm_i_mirror_backslashes): Add prototype.

* libguile/init.c (scm_boot_guile): Call scm_i_mirror_backslashes
  on argv[0].

* libguile/filesys.c (scm_getcwd): Call scm_i_mirror_backslashes
  on the directory name returned by getcwd.

* test-suite/tests/ports.test ("file name separators"): New test.