bug#4719: 23.1; M-& to run commands asynchronously (async-shell-command)

[Daniel Clemente]

I propose to add a new function, async-shell-command, bound to M-&, which in 
analogous to shell-command (bound to M-!) but executes the command in the 
background (as if you had written an ampersand at the end of M-!).
Both functions would respectively correspond to keys ! and & in dired, only 
that ! and & work only in dired but M-! and M-& would be global (work 
everywhere).

The code is in this message in an old thread:
http://article.gmane.org/gmane.emacs.devel/111825

There are still some details to decide: how many buffers to open, where to 
direct STDOUT, whether output should be visible, …

Some modes should be checked since they might be using M-&.