Propagate signals in intermediate bash shells

[lox]

If you execute multiple commands, e.g:

steps:
  -label: "My commands" 
   commands:
     - ./process1
     - ./process2

then we join them together in a single bash shell as /bin/bash -e -c './process1\b./process2, which has the side effect of creating an intermediate shell in between the bootstrap process and your commands. By default, most shells won't automatically propagate signals to subprocesses, so when the job is cancelled the signal gets lost in this intermediate shell.

This could be manually fixed by adding a trap as the first command:

steps:
  -label: "My commands" 
   commands:
     - trap "kill -- \$\$" TERM QUIT INT
     - ./process1
     - ./process2

That ends up kind of noisy and for those not versed in shell voodoo, confusing.

This PR automatically adds this into non-script commands.

[toolmantim]

Noice fix! 👌🏼

[matthewd]

I really like this -- if we're the ones who've decided to inject an extra shell into the process hierarchy, then it follows that we're obliged to ensure it behaves as transparently as possible within our process-management paradigm. 👍🏻

[lox]

Past @lox, just as an FYI, I find trap command in all build output really confusing. The incantation of trap "kill -- \$\$" TERM QUIT INT could probably be hidden in non-debug mode.