feat: allow multiple netprobe addresses

[BPplays]

this PR allows multiple netprobe addresses, to better support IPv6-only, and IPv4-only networks with the same config.

all probes are run concurrently. the first one to succeed cancels the others and causes the main function to stop waiting.

• this PR also changes the netprobe timeout to be context based, given context is used to cancel when a probe succeeds using it for the timeout too makes sense to me
• uses dialer.DialContext with the input host:port pair directly instead of resolving to *net.UDPAddr, this means it will try to resolve the host each time and try to dial any IPs that instead of resolving to just 1 which should improve compatibility with anything that templates/changes the hosts file or any hosts file entries with multiple IPs (i did test and it will resolve from /etc/hosts)
• better error message handling, it now checks a message and only sends it once per probe (it mostly works like retried with better support for specific messages like name resolution errors)
• better handling of netprobe_timeout: it's now tracked with a deadline instead of assuming each iteration is exactly 1s
• there is now just a run-time check for windows specific behavior

P.S. i also didn't commit any of ./vendor for this PR

IPv4 disabled example:

just [9.9.9.9:53]:

-> go run . -config ../config.toml
[2026-09-20 02:35:51] [NOTICE] dnscrypt-proxy 2.1.18
[2026-09-20 02:35:51] [NOTICE] Using default Weighted Power of Two (WP2) load balancing strategy
[2026-09-20 02:35:51] [NOTICE] (9.9.9.9:53) Network not available yet -- waiting...

with [[2620:fe::fe]:53, 9.9.9.9:53]:

-> go run . -config ../config.toml
[2026-09-20 02:36:07] [NOTICE] dnscrypt-proxy 2.1.18
[2026-09-20 02:36:07] [NOTICE] Using default Weighted Power of Two (WP2) load balancing strategy
[2026-09-20 02:36:07] [NOTICE] (9.9.9.9:53) Network not available yet -- waiting...
[2026-09-20 02:36:07] [NOTICE] Network connectivity detected ([2620:fe::fe]:53)
[2026-09-20 02:36:07] [NOTICE] Now listening to [::1]:58003 [UDP]
[2026-09-20 02:36:07] [NOTICE] Now listening to [::1]:58003 [TCP]

[jedisct1]

Before a PR, especially a complicated one that adds unneeded 3rd party dependencies, let's start with a discussion (in the Discussion section) describing the problem you have.

On some operating system (which one?) with network interfaces configured with IPv6 only, netprobe doesn't detect that the network is up?

[BPplays]

@jedisct1 i made a discussion: #3357