Replaces GNU Autotools with CMake, which seems to be taking over the
world of building C projects.

As for dnssim, this makes it (in my opinion) slightly easier to
maintain, as Autotools tend to be quite clunky when settings change,
which is quite often the case when introducing new libraries (like
ngtcp2 for QUIC - !75).

It will also make it kind of easier to use Git submodules for some
dependencies (again handy for ngtcp2, which is not yet stable).

Fix a typo from "DNS-over-TLS POST" to "DNS-over-HTTPS POST"

Add generated .libs directory into .gitignore

This is essentially the opposite of limit-clients.lua.
This new script assigns one client IP address to a single output and
copies all packets for single client into the same output file.

Client assignment is pseudorandom and does not guarantee uniform
assignment. Clients typically do not generate equal load in any metric
(be it QPS or overall resources required to process client's query),
so there would be little point in complicating code for theoretical
uniformity.

Stats generated at the end provide some insight into uniformity of the
split on per-client and per-packet basis.

Ability to stop reading PCAP after specified number of seconds.
It depends on fact that extract-clients script renumbers packets
so first packet has timestamp 0.

Quirk:
Timestamps are checked only after they are already delayed by dnsjit
module. For this reason PCAP reading is in fact terminated when
time for the first packet _after_ the specified time limit passes.
In practice this would be noticeable only if delays between packets
were long.