Handle KeyboardInterrupt gracefully rather the printing a
backtrace. Most users of these tools don't expect a
backtrace when then hit Ctrl-C.

Also, fix a few other inconsistencies found in the python
startup code of these different scripts:
- always call main function 'main' (rather than 'Main')
- always return 0 from main function
- if main takes args never include argv[0]

Review URL: https://codereview.chromium.org/955993006

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294250 0039d316-1c4b-4281-b951-d872f2087c98