Add SIGINFO support on *BSD systems including macOS.

[michael-yuji]

SIGINFO is a signal on *BSD systems to signal the process to output info related messages, for example progress in dd or stack trace in ruby. This PR add SIGINFO support by importing it from libc if available.

Tested platforms: FreeBSD and macOS.

[vorner]

Hello. Can you have a look and also extend the definitions in src/low_level/signal_details.rs?

Anyway, „support“ is a bit overstatement here. It's just convenience re-export. Whole signal-hook should work fine if you just pass the libc::SIG* constant or just the number.

[codecov-commenter]

Codecov Report

Merging #133 (f3102f3) into master (c7c68d4) will decrease coverage by 0.87%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #133      +/-   ##
==========================================
- Coverage   68.75%   67.88%   -0.88%     
==========================================
  Files          16       16              
  Lines         685      685              
  Branches      104      104              
==========================================
- Hits          471      465       -6     
- Misses        132      139       +7     
+ Partials       82       81       -1     

Impacted Files	Coverage Δ
src/lib.rs	100.00% <ø> (ø)
src/low_level/signal_details.rs	17.50% <ø> (ø)
signal-hook-registry/src/half_lock.rs	75.00% <0.00%> (-6.67%)	⬇️
src/low_level/channel.rs	76.08% <0.00%> (-2.18%)	⬇️

[michael-yuji]

@vorner Thanks for responding and the tip, just updated.