Add --pause to space out file downloads by a random delay

[xroche]

A new --pause MIN[:MAX] (seconds, -%G) waits a random delay between MIN and MAX before each file, so a crawl looks less like a bot and is gentler on the server. A single value (--pause 8) is a fixed delay; off by default. Issue #185.

It reuses the existing non-blocking launch gate, back_pluggable_sockets_strict, the same one that enforces --connection-per-second. A blocking Sleep() would be wrong here: downloads run through one select() pump over several sockets, so sleeping would freeze the in-flight transfers on the other sockets. The gate instead just withholds new file launches until the delay elapses, one file per gap, while the existing transfers keep draining.

The per-gap delay is derived from the last-request timestamp rather than a fresh rand() per call. The gate is evaluated many times within one gap, so sampling on every pass would fire as soon as the smallest roll dipped below the elapsed time, biasing the real delay toward MIN. A value that stays constant between launches keeps the target stable within a gap and rerolls it on the next launch, with no scratch state to store.

Two int fields are appended at the httrackp tail, so the exported ABI is unchanged and there's no soname bump (same shape as the --cookies-file change). The pause is global across the crawl, matching how --connection-per-second already behaves.

Tests cover the delay function directly (range and spread, with teeth against the bias-toward-MIN bug, seeded with consecutive-millisecond values like real launch timestamps), a copy_htsopt round-trip, command-line validation including non-finite input such as nan, and a local-server crawl that asserts the pause measurably slows a multi-file mirror.

Closes #185