pkg/parsers/kernel: simplify

Since golang.org/x/sys/unix provides portable way to use uname(2)
syscall, let's use it for all unix variants, removing the duplicated
code from uname_*.go.

While at it:
 - remove uname_unsupported*.go (not used);
 - use unix.ByteSliceToString.

NOTE this also removes the public definition of struct Utsname. I am
puzzled as to why it was exported as it is not (and was not) used by
any public functions in this package, nor I was able to find any usage
of kernel.Utsname (using sourcegraph).

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

pkg/parsers/kernel/kernel_unix.go

@@ -1,35 +1,21 @@
-//go:build linux || freebsd || solaris || openbsd
-// +build linux freebsd solaris openbsd
-
 // Package kernel provides helper function to get, parse and compare kernel
 // versions for different platforms.
 package kernel
 
 import (
-	"bytes"
-
 	"github.com/sirupsen/logrus"
+	"golang.org/x/sys/unix"
 )
 
 // GetKernelVersion gets the current kernel version.
 func GetKernelVersion() (*VersionInfo, error) {
-	uts, err := uname()
-	if err != nil {
-		return nil, err
-	}
+	uts := &unix.Utsname{}
 
-	release := make([]byte, len(uts.Release))
-
-	i := 0
-	for _, c := range uts.Release {
-		release[i] = byte(c)
-		i++
+	if err := unix.Uname(uts); err != nil {
+		return nil, err
 	}
 
-	// Remove the \x00 from the release for Atoi to parse correctly
-	release = release[:bytes.IndexByte(release, 0)]
-
-	return ParseRelease(string(release))
+	return ParseRelease(unix.ByteSliceToString(uts.Release[:]))
 }
 
 // CheckKernelVersion checks if current kernel is newer than (or equal to)