Added all signals, exit with child exit code, reap all remaining pid's

[bartmeuris]

This pull request contains the following changes:

• added all signals mentioned in the docker/pkg/signal/signal_linux.go file (with the exception of SIGKILL and SIGSTOP), which should now be passed to the child process.
• After the child process exits, I loop over the waitpid(-1, NULL, WNOHANG) to avoid child processes of your child process not being reaped.
• return the child processes' exit code by returning WEXITSTATUS(cstatus);

[rciorba]

Thanks for sending this! I added some comments inline.

Also, could you create a CONTRIBUTORS.txt and add your name to it? Something along the lines of: Author Name - Handling of return code and proper signal handling.