core: Add current-output-port, open-output-file, set-current-output-port.

core: Add current-output-port, open-output-file, set-current-output-port.

* libc/include/fcntl.h: Declare it.  Add some fcntl defines.
* libc/include/stdio.h: Remove fcntl defines, Declare g_stdout.
* module/language/c99/compiler.mes (c99-input->ast): Define O_WRONLY, O_RDWR.
* module/mes/guile.mes (with-output-to-file, with-output-to-port): New functions.
* src/posix.c (current_output_port, open_output_file,
  set_current_output_port): New functions.
* libc/mlibc.c (open): Add optional mode parameter.
* module/mes/libc-i386.mes (i386:open): Forward third parameter.
* scaffold/mini-mes.c (main): Init g_stdout.
* src/mes.c (main): Likewise.