Warn if running with root privileges

[stevenengler]

See https://shadow.github.io/docs/guide/compatibility_notes.html#notes-6 for an example where this causes issues:

Notes

1, Shadow must be run as a non-root user since opentracker will attempt to drop privileges if it detects that the effective user is root.

[codecov]

Codecov Report

Base: 68.01% // Head: 67.71% // Decreases project coverage by -0.30% ⚠️

Coverage data is based on head (05691bd) compared to base (c886d81).
Patch coverage: 0.00% of modified lines in pull request are covered.

❗ Current head 05691bd differs from pull request most recent head 28acbf6. Consider uploading reports for the commit 28acbf6 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2670      +/-   ##
==========================================
- Coverage   68.01%   67.71%   -0.30%     
==========================================
  Files         202      202              
  Lines       30315    30308       -7     
  Branches     5919     5924       +5     
==========================================
- Hits        20619    20524      -95     
- Misses       5027     5123      +96     
+ Partials     4669     4661       -8     

Flag	Coverage Δ
tests	67.71% <0.00%> (-0.30%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/main/core/main.rs	39.13% <0.00%> (-1.00%)	⬇️
src/main/host/syscall/handler/eventfd.rs	0.00% <0.00%> (-69.45%)	⬇️
src/main/host/descriptor/eventfd.rs	8.08% <0.00%> (-56.57%)	⬇️
src/main/core/worker.rs	79.27% <0.00%> (-2.77%)	⬇️
src/main/host/syscall/type_formatting.rs	74.78% <0.00%> (-2.61%)	⬇️
src/main/core/scheduler/pools/bounded.rs	74.50% <0.00%> (-1.97%)	⬇️
src/main/host/syscall/handler/mod.rs	77.77% <0.00%> (-0.86%)	⬇️
src/main/host/descriptor/pipe.rs	83.75% <0.00%> (-0.63%)	⬇️
src/lib/shadow-shim-helper-rs/src/rootedcell/rc.rs	94.64% <0.00%> (-0.60%)	⬇️
src/main/host/syscall/handler/socket.rs	68.43% <0.00%> (-0.43%)	⬇️
... and 10 more

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