Use dynamic registration

[lbartnik]

• switch to the recommended way of registering native routines (https://cran.r-project.org/doc/manuals/R-exts.html#Registering-native-routines)
• update man pages with roxygen 6

[codecov-io]

Codecov Report

Merging #47 into master will not change coverage.
The diff coverage is n/a.

@@         Coverage Diff          @@
##           master   #47   +/-   ##
====================================
  Coverage      80%   80%           
====================================
  Files          11    11           
  Lines         655   655           
====================================
  Hits          524   524           
  Misses        131   131

Impacted Files	Coverage Δ
src/sub-linux.cc	70.4% <ø> (ø)	⬆️
src/rapi.cc	82.35% <ø> (ø)	⬆️
R/subprocess.R	100% <ø> (ø)	⬆️

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