Capped percent complete in monitor.c

[phillip-stephens]

Resolves #823

Saw with an assert that sometimes, the remaining_secs was < 0, causing our % remaining calculation to go over 100.

Mar 15 19:48:19.739 [WARN] monitor: rest = 2.043692
zmap: /home/pstephens/zmap-dev/src/monitor.c:165: compute_remaining_time: Assertion `zconf.cooldown_secs - (now() - zsend.finish) >= 0' failed.

I'm not entirely sure how this is happening, but this fix seems both relatively reasonable and sufficient to prevent it happening again.

[phillip-stephens]

LMK what you think @zakird or if you see an alternative, nothing immediately sprung to mind.

[zakird]

We should change to loop once (call min_d only once)

[phillip-stephens]

Addressed!