bug#23206: 25.0.92; dired-shell-stuff-it: wait until all parallel jobs finish

[Tino Calancha]

I must clarify the tittle of this thread (the word wait).
Of course, the process is still asynchronous! :-)

The command wait just avoid `shell-command-sentinel' be triggered
too early.  Once all jobs in the background finish, the shell
returns with the return code of the last command in the list; then
`shell-command-sentinel' do its stuff.

The command 'wait' is specified by posix.
I have verified that it is present in following shells:

bash: 4.3, 3.2
dash: 0.5.8-2.2
posh: 0.12.6
bsd-csh: 1.65 (20110502-2.1)
tcsh: 6.18, 6.14
ksh: 93u+ 2012-08-01,  20100202-1
zsh: 5.2, 4.2