Use cleancall to solve a file descriptor leak

[gaborcsardi]

We can use it for better resource cleanup in general.

[codecov-io]

Codecov Report

Merging #238 into master will decrease coverage by 1.82%.
The diff coverage is 69.13%.

@@            Coverage Diff             @@
##           master     #238      +/-   ##
==========================================
- Coverage   75.59%   73.77%   -1.83%     
==========================================
  Files          36       32       -4     
  Lines        3749     2955     -794     
==========================================
- Hits         2834     2180     -654     
+ Misses        915      775     -140

Impacted Files	Coverage Δ
src/init.c	100% <100%> (ø)	⬆️
R/process.R	78.44% <100%> (-2.77%)	⬇️
R/cleancall.R	100% <100%> (ø)
R/errors.R	81.31% <45.45%> (-4.94%)	⬇️
src/cleancall.c	70.31% <70.31%> (ø)
R/utils.R	59.5% <0%> (-25.62%)	⬇️
src/client.c	36.61% <0%> (-19.17%)	⬇️
src/create-time.c	50% <0%> (-18.58%)	⬇️
R/run.R	82.12% <0%> (-10.06%)	⬇️
... and 10 more

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