Use ulimit to check the open files soft limit

[tagatac]

What is changing: Use the value returned from ulimit -n as the open files soft limit instead of the value returned from the syscall getrlimit.

Why this change is being made: Since Go 1.19, during initialization, Go programs unconditionally set a high open files soft limit for themselves without modifying the system-wide defaults. This soft limit also does not apply to subprocesses run via the exec package. Because wkhtmltopdf is run as a subprocess, the soft limit set by the Go program during initialization does not give us any information about the limits that will be applied to wkhtmltopdf. Two approaches would be to

1. always call setrlimit on every run so that the soft limit will apply to subprocesses, or
2. use ulimit to check the soft limit that will be applied to wkhtmltopdf.

I chose to implement approach 2. to reduce the number of syscalls.

More details:

• https://go.dev/src/syscall/rlimit.go
• runtime: automatically bump RLIMIT_NOFILE on Unix golang/go#46279
• https://stackoverflow.com/q/73640931/5403337
• https://www.perplexity.ai/search/explain-why-golang-returns-an-tho.xk6ARhOs6ZSPr4kx8A

Related issue(s): Fixes #63

Follow-up changes needed: None

Is the change completely covered by unit tests? If not, why not?: Yes

[codecov]

Codecov Report

Attention: Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.

Project coverage is 93.98%. Comparing base (8d1118a) to head (7d8af1b).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/bagoup/main.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #66      +/-   ##
==========================================
+ Coverage   93.76%   93.98%   +0.21%     
==========================================
  Files          14       14              
  Lines         786      798      +12     
==========================================
+ Hits          737      750      +13     
+ Misses         45       44       -1     
  Partials        4        4              

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