Allow pipes in commands

[theomega]

The current implementation does not allow to specify pipes as commands.

So watch 'ls | grep txt' fails, even worse without a clear error-message:

waitpid(): No child processes
exit: 1

This comes from the fact that the old watch implementation uses the system function to execute the commands. This function uses a shell which handles the pipes. execvp does not support pipes in its commands, you would have to set them up by yourself.

The following patch changes some things:

• Per default execute thru system
• Add a -x/--exec flag which switches back to the old behaviour
• Output a correct error-message on execvp errors.

This makes watch more usable and more compliant to the original implementation.

[tj]

haha good call with system, I didn't even think about pipelines for some reason. I'll check it out tomorrow