It often fails in CI.

Making the "vet" target will run "go vet ./..." for each supported
GOOS.

Also, some of the warnings produced by go 1.15 vet were mitigated.

TestGetProcInodesAll: create a server so there are some opened inodes
TestUsers: skip if Users is empty, because of an empty /var/run/utmp
Test_Process_Groups: skip if Groups is empty
TestConnectionsMax: skip on CI, not only CircleCI

Changes the port parsing from `/proc/net/*` files records from parsing them as 64-bit integers to parse them as 16-bit unsigned integers.

While this is mostly a cosmetic change, it will also make so that the code fails faster in case the entry is malformed (for whatever reason).

Note that the returned value is still casted to uint32 when an  `Addr` object is created.
It seems to me that the `Addr.port` field should be changed to `uint16` but maybe some other APIs/systems wants it to be `uint32` and also changing it there may require changes in users code if they update. This being said I am not changing that field's type.

These funcs are only used in net_unix.go which is only compiled for
darwin and freebsd.

`connectionsList, err := net.ConnectionsMax("tcp4", 1000)`
when you run net.ConnectionsMax,you will find some proc is not equal with the `netstat -lptn`

exec.LookPath searches for the file in the $PATH, which mean giving it an absolute path is against it's own purposes.

Signed-off-by: NJoe Stringer <joe@cilium.io>

Found using https://go-critic.github.io/overview#emptyFallthrough-ref

Might be useless because of https://docs.microsoft.com/en-us/windows/desktop/dlls/dynamic-link-library-search-order#search-order-for-desktop-applications
but better be safe than sorry.

Ref: https://github.com/shirou/gopsutil/issues/570#issuecomment-413951653

Signed-off-by: NKamil Dziedzic <arvenil@klecza.pl>

The order of init function execution is dependant on the order that the
source files are passed to the compiler. This causes issues when
building under other build systems, such as bazel or buck, as they are
not guarenteed to maintain the same file order as the default go tool.

This retrieves open TCP/UDP connections by using netstat(1)
File descriptors and pids are not supported.

This fixes #433