Fixes #1540

Problem

The -c flag (general concurrency) did not affect DNS resolution.
When scanning with a host list (-l), DNS lookups were performed sequentially, leading to slow performance with large input files.

Example:

$ time ./naabu -l domains.txt -p 80 -c 100
# Duration almost the same as -c 25

Root Cause

DNS resolution inside PreProcessTargets() was always bound to Threads (-c).
There was no dedicated concurrency control for DNS lookups, so the -c flag only applied to scanning, not to DNS resolution.

Changes

• Added a new CLI flag: --dns-concurrency

  • Controls the number of concurrent DNS resolution workers.
  • Defaults to -c when not explicitly set.

• Updated options.go to parse the new flag.

• Updated targets.go to use a sized wait group based on DnsConcurrency instead of Threads during target preprocessing.

• Updated help text and usage docs.

Result

Before:

$ time ./naabu -l domains.txt -p 80 -c 25
real    0m3.44s

$ time ./naabu -l domains.txt -p 80 -c 100
real    0m3.42s   # no improvement

After:

$ time ./naabu -l domains.txt -p 80 -c 25 --dns-concurrency 100
real    0m1.72s   # faster resolution with higher DNS concurrency

This makes DNS resolution behavior consistent with user expectations and improves scan speed for large host lists.