enhance 'ipfs diag sys' output

We like to extend the `ipfs diag sys` output to include more system information than just the platform, architecture, and the go version.

Currently, a typical output looks like this:

```
{
"diskinfo": {
"free_space": 18522890240,
"fstype": "2435016766",
"total_space": 14918279168
},
"environment": {
"GOPATH": "",
"IPFS_PATH": ""
},
"ipfs_commit": "",
"ipfs_version": "0.6.0-dev",
"memory": {
"swap": 15964000,
"virt": 5207176000
},
"net": {
"interface_addresses": [
"/ip4/127.0.0.1",
"/ip4/192.168.8.2",
"/ip6/::1",
"/ip6/fe80::8c71:2b0e:6947:3090"
],
"online": true
},
"runtime": {
"arch": "amd64",
"compiler": "gc",
"gomaxprocs": 4,
"numcpu": 4,
"numgoroutines": 1864,
"os": "linux",
"version": "go1.14.4"
}
}
```

@Stebalien likes to remove the interface_addresses from the output.

I like to add some more information, like those commands provide:

```
Linux:
uname -a; grep -Hv "^#" /etc/*release
BSD:
uname -a; uname -K
MacOS:
uname -a; sw_vers
```

_This idea was discussed earlier in the PR https://github.com/ipfs/go-ipfs/pull/7408_