Avoid stdio in SIGINT handler

Avoid stdio in SIGINT handler

* admin/merge-gnulib (GNULIB_MODULES): Add ignore-value.
* lib/gnulib.mk, m4/gnulib-comp.m4: Regenerate.
* lib/ignore-value.h: New file, from gnulib.
* src/keyboard.c: Include it.
(write_stdout, read_stdin): New functions.
(handle_interrupt): Use them instead of printf and getchar,
and avoid fflush when handling signals.