Fix format warnings of type 'pid_t'

[Low-power]

Solaris has defined type pid_t as long int on 32-bit platforms, so using %d in a format string for pid_t will cause a compiler warning about the type mismatch.

[codecov-io]

Codecov Report

Merging #53 into master will increase coverage by 0.06%.
The diff coverage is 40%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   50.26%   50.32%   +0.06%     
==========================================
  Files          33       33              
  Lines       13432    13432              
==========================================
+ Hits         6751     6760       +9     
+ Misses       6681     6672       -9

Impacted Files	Coverage Δ
lib/facil/redis/redis_engine.c	0% <0%> (ø)	⬆️
lib/facil/fio.c	58.82% <46.15%> (+0.19%)	⬆️

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[boazsegev]

Interesting! Thanks for the PR 👏🏻☺️🙏🏻

Question... wouldn't long int have a higher range? how high do Solaris process number go?

Maybe we should change the printout upwards (use %zd and cast all pid_t to ssize_t)...?

[Low-power]

Nope, long int has same size with int on Solaris i386 (32-bit).