When debugging shell scripts that use '@', I sometimes need to know what is happening.  Using -n tells me what will happen next time I run it, but not what is happening right now when the failure happens.  I've added a --print_all_cmds option (-P) that prints out any shell command as it's executing, ignoring '@', .SILENT, etc.  Doc update is included with the attached patch, and the doc explains the usage model that gets me here (which, briefly, is to use @ to hide complexities that don't change so I can more easily see the ones that do -- I know others who use this style, although it is admittedly a minority one).  This has helped me debug a lot of makefile stuff, and so seems quite useful.  Tests for the new option are also included.

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