feat: replace custom sysctl implementation with stdlib

[kruskall]

Remove custom string->mib conversion code and sysctl implementation.
Reduce code complexity and cgo dependency

Related to elastic/apm-server#8718

[elasticmachine]

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• Start Time: 2022-09-14T17:28:16.307+0000

• Duration: 8 min 30 sec

Test stats 🧪

Test	Results
Failed	0
Passed	98
Skipped	2
Total	100

[andrewkroh]

The one concern I had was with performance. The only difference is that the unix package makes two sysctl calls (one to determine buf size and one to get the data). The old implementation in this package made one call with a max size buffer.

This does make it a bit slower, but it's not significant.

func BenchmarkKernProcargs(b *testing.B) {
	pid := os.Getpid()
	var p process
	for i := 0; i < b.N; i++ {
		kern_procargs(pid, &p)
	}
}

% benchcmp before-self.txt after-self.txt                                                      
benchcmp is deprecated in favor of benchstat: https://pkg.go.dev/golang.org/x/perf/cmd/benchstat
benchmark                    old ns/op     new ns/op     delta
BenchmarkKernProcargs-10     13783         15075         +9.37%

benchmark                    old allocs     new allocs     delta
BenchmarkKernProcargs-10     106            109            +2.83%

benchmark                    old bytes     new bytes     delta
BenchmarkKernProcargs-10     31931         34104         +6.81%

% benchstat before-self.txt after-self.txt
name             old time/op    new time/op    delta
KernProcargs-10    13.8µs ± 0%    15.1µs ± 0%   ~     (p=1.000 n=1+1)

name             old alloc/op   new alloc/op   delta
KernProcargs-10    31.9kB ± 0%    34.1kB ± 0%   ~     (p=1.000 n=1+1)

name             old allocs/op  new allocs/op  delta
KernProcargs-10       106 ± 0%       109 ± 0%   ~     (p=1.000 n=1+1)

[andrewkroh]

And can you please add changelog entries for this and the other recent changes.

[kruskall]

Removed unused methods and added the changelog entry 👍