Allow compilation with `--disable-posix'.

Allow compilation with `--disable-posix'.

Reported by Dmitry Dzhus <dima@dzhus.org>.

* configure.ac: Remove `AC_LIBOBJ([filesys])'.  Document
  `--disable-posix' as omitting non-essential POSIX interfaces.

* libguile/Makefile.am (libguile_@GUILE_EFFECTIVE_VERSION@_la_SOURCES):
  Add `filesys.c'.
  (DOT_DOC_FILES): Add `filesys.doc'.
  (EXTRA_libguile_@GUILE_EFFECTIVE_VERSION@_la_SOURCES): Remove
  `filesys.c'.

* libguile/posix.c (scm_mkstemp, scm_access): Move to `filesys.c'.
  (scm_init_posix): Move `R_OK' etc. to `filesys.c'.

* libguile/filesys.c (scm_chown, scm_chmod, scm_umask, scm_open_fdes,
  scm_open, scm_close, scm_close_fdes, scm_link, scm_tc16_dir,
  scm_directory_stream_p, scm_opendir, scm_readdir, scm_rewinddir,
  scm_closedir, scm_dir_print, scm_dir_free, scm_chdir, scm_getcwd,
  set_element, fill_select_type, get_element, retrieve_select_type,
  scm_select, scm_fcntl, scm_fsync, scm_symlink, scm_readlink,
  scm_lstat, scm_copy_file): Conditionalize on HAVE_POSIX.
  (scm_mkstemp, scm_access): New functions.
  (scm_init_filesys): Conditionalize `scm_tc16_dir', `O_RDONLY', etc. on
  HAVE_POSIX.  Define `R_OK', `W_OK', etc.

* libguile/fports.c (fport_print): Use `scm_ttyname' only when
  HAVE_POSIX.

* libguile/i18n.c (lock_locale_mutex, unlock_locale_mutex): New
  functions.  Update users of `scm_i_locale_mutex' to use them.

* libguile/init.c (scm_i_init_guile): Always call `scm_init_filesys'.

* meta/guile-tools.in (main): Use `setlocale' only when it is defined.

* module/ice-9/boot-9.scm: Always load `ice-9/posix'.