Add support for socketpair syscall

[robgjansen]

Passes all existing socketpair tests.

refs #849

[codecov]

Codecov Report

Merging #1058 (41e0c30) into dev (b963f5e) will increase coverage by 0.25%.
The diff coverage is 90.58%.

@@            Coverage Diff             @@
##              dev    #1058      +/-   ##
==========================================
+ Coverage   54.82%   55.07%   +0.25%     
==========================================
  Files         127      127              
  Lines       19177    19231      +54     
  Branches     4585     4592       +7     
==========================================
+ Hits        10513    10592      +79     
+ Misses       5898     5871      -27     
- Partials     2766     2768       +2     

Flag	Coverage Δ
tests	55.07% <90.58%> (+0.25%)	⬆️

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Impacted Files	Coverage Δ
src/main/host/descriptor/descriptor_table.c	68.35% <0.00%> (+2.10%)	⬆️
src/main/host/descriptor/transport.c	64.28% <ø> (ø)
src/main/host/syscall/uio.c	0.00% <ø> (ø)
src/main/host/descriptor/channel.c	65.47% <66.66%> (+4.76%)	⬆️
src/main/host/syscall/socket.c	73.83% <92.20%> (+1.77%)	⬆️
src/main/host/syscall/unistd.c	57.30% <100.00%> (ø)
src/main/host/syscall_handler.c	53.11% <100.00%> (+0.22%)	⬆️
src/shim/preload_syscalls.c	47.76% <100.00%> (+0.23%)	⬆️
src/main/core/slave.c	68.95% <0.00%> (+0.65%)	⬆️
src/main/host/thread_ptrace.c	66.61% <0.00%> (+0.87%)	⬆️
... and 2 more

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[sporksmith]

FWIW I don't know of a reason why we'd want that