Check the return value of libc's functions to make `_FORTIFY_SOURCE=2' work.

Check the return value of libc's functions to make `_FORTIFY_SOURCE=2' work.

This fixes bug #24009 reported by Martin Pitt.

* libguile/threads.c (guilify_self_1): Check the return value of
  pipe(2).
  (scm_std_select): Use `full_read ()' instead of `read ()' when reading
  from WAKEUP_FD.

* libguile/async.c (scm_i_queue_async_cell): Use `full_write ()' instead
  of write(2) when writing to SLEEP_FD.

* libguile/fports.c (fport_flush): Likewise.

* libguile/posix.c (getgroups): Use the return value of getgroups(2) as
  NGROUPS.
  (scm_nice): Get the return value of nice(2) to make glibc happy.

* libguile/scmsigs.c (take_signal): Use `full_write ()' instead of
  write(2).