makepkg: correctly handle missing download clients

This was broken in commit 882e707e40,
which changed 'plain()' messages to go to stdout, which was then
captured as the download client in question: cmdline=("Aborting...").

The result was a very confusing error message e.g.

/usr/share/makepkg/source/file.sh: line 72: $'\E[1m': command not found

or with makepkg --nocolor:

/usr/share/makepkg/source/file.sh: line 72: Aborting...: command not found

The problem here is that we checked to see if an asynchronous subshell,
in our case <(...), failed, by checking if its captured stdout is
non-empty. Which is terrible, and also a limitation of old bash. But
bash 4.4 can use wait $! to retrieve the return value of an asynchronous
subshell. Now we target that as our minimum, we can sanely handle errors
in such functions.

Losing error messages on stdout by capturing them in a variable instead
of printing them, continues to be a problem, but this will be fixed
systematically in a later commit.

Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Allan McRae <allan@archlinux.org>
(cherry picked from commit 381e113755)

scripts/libmakepkg/source/file.sh.in

@@ -42,7 +42,7 @@ download_file() {
 	# find the client we should use for this URL
 	local -a cmdline
 	IFS=' ' read -a cmdline < <(get_downloadclient "$proto")
-	(( ${#cmdline[@]} )) || exit
+	wait $! || exit
 
 	local filename=$(get_filename "$netfile")
 	local url=$(get_url "$netfile")