ls: allow interruption when reading slow directories

ls: allow interruption when reading slow directories

Postpone installation of signal handlers until they're needed.
That is right before the first escape sequence is printed.

* src/ls.c (signal_setup): A new function refactored from main()
to set and restore signal handlers.
(main): Move signal handler setup to put_indicator()
so that the default signal handling is untouched as long as possible.
Adjusted condition for restoring signal handlers to reflect the change.
(put_indicator): Install signal handlers if called for the very first
time.  It uses the same code that was in main() prior to this commit.
* NEWS: Mention the improvement.

See https://bugzilla.redhat.com/1365933
Fixes http://bugs.gnu.org/24232