Initial implementation of HostInfo for FreeBSD platforms.

On some platforms (e.g. FreeBSD) user home directories are symlinks
and os.Getcwd will return the symlink path.

On FreeBSD CWD for processes that's not the running process will not
give the symlink but the resolved path causing the test to fail at this
point.

Fix is to use the os.SameFile call to resolve both Os.Getcwd and CWD.

Implement Process(), Self() and Processes() to the bare minimum.
That is passing all basic tests.

- Fix issue on FreeBSD 12.
- Minor cosmetic issues.

- Remove cgo build directives from things not needing it.
- Add linker flags into .go files.

[git-generate]

gofumpt -w --extra $(find . -name '*.go')

providers/freebsd/host_freebsd.go:56:9: cannot use newHost() (value of type *host) as types.Host value in return statement: *host does not implement types.Host (missing method FQDN)

Replace usages of github.com/pkg/errors and github.com/joeshaw/multierror.

Make sure that all code depending on cgo is tagged appropriately.

Replace "syscall" with golang.org/x/sys/unix as per the recommendation of
the stdlib syscall package for new code.

Add a cross-compile sanity check for freebsd/amd64.

The baseDir was never used so implement this directly within OperativingSystem().

Prior to this change the used memory value looked incorrect:

{
  "host.memory": {
    "total_bytes": 6402035712,
    "used_bytes": 6402035712,
    "available_bytes": 162770944,
    "free_bytes": 0,
    "virtual_total_bytes": 1073610752,
    "virtual_used_bytes": 0,
    "virtual_free_bytes": 1073610752
  }
}

Combine all non-cgo sysctl based data sources into one file
to improve the code locality.

Remove vmstat.go and vmstat_test.go for now because these were unused.
They can be added back later in a separate PR.

ioutil is deprecated.

  === RUN   TestHost
      system_test.go:309: error parsing kern.cp_time: strconv.ParseUint: parsing "\xd8\x0f\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00Q\x05\x00\x00\x00\x00\x00\x00\xce\x01\x00\x00\x00\x00\x00\x00k": invalid syntax

       system_test.go:265:
          	Error Trace:	system_test.go:265
          	Error:      	Received unexpected error:
          	            	open 1620/net/snmp: no such file or directory
          	Test:       	TestSelf

The implementation was not tested and did not work.

I don't think FreeBSD has /proc/<pid>/net/{snmp,netstat} paths.

Based on the timeval definition, I think the conversion to time.Duration was incorrect.

https://elixir.bootlin.com/freebsd/v14.0/source/sys/sys/_timeval.h#L47

https://elixir.bootlin.com/freebsd/v14.0/source/sys/sys/user.h#L153

…ARCH) values

Requested upstream changes