Use `socket` instead of `gen_udp`

[hauleth]

This change adds support for sending file descriptors to the systemd, to keep them open between application restarts.

I haven't tested it fully yet, but unfortunately this feature will not fully work right now as Erlang do not allow for "leaking" the opened sockets when controlling process dies (and at the same time - when the VM dies).

[codecov[bot]]

Codecov Report

Merging #20 into master will decrease coverage by 0.44%. The diff coverage is 92.30%.

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
- Coverage   95.71%   95.27%   -0.45%     
==========================================
  Files           8        8              
  Lines         280      296      +16     
==========================================
+ Hits          268      282      +14     
- Misses         12       14       +2     

Impacted Files	Coverage Δ
src/systemd_sup.erl	96.87% <50.00%> (ø)
src/systemd_socket.erl	86.36% <87.50%> (-3.64%)	:arrow_down:
src/systemd.erl	93.22% <100.00%> (+0.76%)	:arrow_up:
src/systemd_journal_h.erl	96.36% <100.00%> (+0.03%)	:arrow_up:
src/systemd_kmsg_formatter.erl	100.00% <100.00%> (ø)
src/systemd_watchdog.erl	94.73% <100.00%> (-0.92%)	:arrow_down:

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