Implement full context support in ZDNS and better SIGTERM/SIGINT handling

[phillip-stephens]

On Mac, there's always been an annoying behavior (existing back to at least v1.1.0 that I saw) where SIGINT (ctrl + C) wouldn't kill ZDNS. I've had ZDNS continue after SIGINT for multiple minutes, and I end up just closing the terminal and opening another. Linux doesn't seem to have this behavior and ZDNS will exit immediately.

This PR implements both:

• Better handling of context.Context's in ZDNS. Now the main API's (ExternalLookup(), etc.) all accept a ctx object.
• Better handling of SIGINT when using ZDNS as a CLI tool. Now, a goroutine will watch for SIGINT and SIGTERM and cancel the main context.Context. This ensures ZDNS exits quickly on Mac.

Limitation

2 Second Delay in Cancellation

With this PR, ZDNS on Mac continues for 2-3 seconds post SIGINT. Using stack dumps, I noticed all goroutines are stuck in a zmap/dns lookup. At first this was really surprising since all the dns lookups we do use a context object which should be canceled when the main context is. However, looking closer at the source code for zmap/dns, only the ctx.Deadline is used to configure timeouts on the DNS connection object, and our default --network-timeout is 2s. So we can't cancel in-progress DNS requests and will have to let them expire. Regardless, this seems strictly better than our current state.

Final Status Update

Currently, the status handler will print a final Scan Complete; X names scanned... with SIGINT. Wasn't sure if it'd be better to just immediately quit or print this.

Update - Per PR discussion, we now print a Scan Aborted instead of Scan Complete to indicate to the user we aborted due to their signal and didn't exit cleanly.

Testing

v1.1.0 Behavior

Screen.Recording.2025-04-10.at.4.34.02.PM.mov

This PR Behavior

Screen.Recording.2025-04-15.at.2.27.19.PM.mov

[phillip-stephens]

@zakird Wondering if you have thoughts on the Final Status Update Limitation I listed in the description? Otherwise this is good to go.

[zakird]

Currently, the status handler will print a final Scan Complete; X names scanned... with SIGINT. Wasn't sure if it'd be better to just immediately quit or print this.

I don't think it super matters. If someone killed the process, they shouldn't really rely on the results. I'm fine leaving it has having it print the status message, but I thought that you might want to print something afterwards that indicates that the process has been interrupted so that the user doesn't inadvertently think that the process completed successfully.