Add intercept hook after kernel syscall completes

[alberto-miranda]

This PR adds an additional intercept_hook_point_post_kernel hook to the API that allows users to register a function to be called after a system call has been executed by the kernel by using syscall_no_intercept. In our case, this allows us to debug our interception layer, since we can now keep track of/log system call results for those syscalls where we don't really need to do anything. Since this has proved really useful to us, we thought it might be a worthwhile addition to the API.

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This change is 

[alberto-miranda]

This PR doesn't include a specific test for the new hook since I was unsure what the best way to test it was. I'm open to suggestions :)

[codecov-commenter]

Codecov Report

Merging #103 (a526e12) into master (7031a8f) will decrease coverage by 0.08%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #103      +/-   ##
==========================================
- Coverage   91.95%   91.86%   -0.09%     
==========================================
  Files          11       11              
  Lines         957      959       +2     
==========================================
+ Hits          880      881       +1     
- Misses         77       78       +1     

Flag	Coverage Δ
pmemfile_tests	86.28% <50.00%> (-0.08%)	:arrow_down:
regular_tests	90.71% <50.00%> (-0.09%)	:arrow_down:

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Impacted Files	Coverage Δ
include/libsyscall_intercept_hook_point.h	50.00% <ø> (ø)
src/intercept.c	80.37% <50.00%> (-0.39%)	:arrow_down:

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