This reverts commit 2f1edeff.

In #1061 we changed the default command to retry with a simpler find
command with fewer arguments if the first find command failed. This was
to support stripped-down verions of find that do not support -fstype
argument.

However, this caused an unwanted side-effect of yielding duplicate
entries when the first command failed after producing some lines.

We revert the change in this commit, so the default command will not
work with find without -fstype support. But we now print better error
message in that case so that the user can set up a working
$FZF_DEFAULT_COMMAND.

Close #1120 #1167

- Use bash for `set -o pipefail`
- Fall back to simpler find command when the original command failed

Related: #1061

Instead of notifying the event coordinator (EventBox) whenever a new
line is arrived, start a background goroutine that periodically does the
task. Atomic.StoreInt32 is much cheaper than mutex synchronization
that happens during EventBox update.

Close #818

`TAGS=termbox make` (or `go build -tags termbox`)

Use hard-coded limit to keep it simple. An alternative is to dynamically
calculate the width of the visible area and use it as the limit, but it
can cause unwanted truncation of the query on screen resize/split.