Signal handling meta-issue

[pvdrz]

This issue tracks if sudo-rs behaves the same as ogsudo when receiving certain signals:

• SIGALRM: Tracked by sudo should terminate the command when receiving SIGALRM #448
• SIGCHLD: This is indirectly tested by the fact that we handle SIGCHLD by waiting for child processes but we shouldn't misbehave if someone sends us a fake SIGCHLD.
• SIGCONT
• SIGHUP
• SIGINT
• SIGKILL: It is not possible to handle this signal but we shouldn't leave child processes hanging around.
• SIGQUIT
• SIGSTOP: It is not possible to handle this signal
• SIGTSTP: Tracked by sudo-rs ignores SIGTSTP signal #325
• SIGTERM
• SIGTTIN
• SIGTTOU
• SIGUSR1
• SIGUSR2
• SIGWINCH

This should be tested with and without a pty for every process that sudo creates.