Propagate SIGTSTP (Ctrl-Z) to children

[raphCode]

This is basically #30 re-occuring, at least for me Ctrl Z did not stop the spawned process:

• watchexes yes
• Press Ctrl-Z
• Observe how terminal is still spammed
• Blindly type kill %% and press Enter (assuming bash)
• spamming stops
• [1]+ Done watchexec yes appears after the next Enter

OS: Arch Linux watchexec 1.20.5

[matu3ba]

Same reason applies as #387: Handling signaling graceful in all cases is complex.