fix build-tags, fix/update deprecated features, and fix inconsistency in path used

[thaJeztah]

I split the changes in "focussed" commits, but can open some of these as separate PRs if preferred. See individual commits for details:

remove redundant build-tag comments

• The "linux" build tags were redundant as they were in a _linux file
• The project already states that go1.10 is a minimum (and long obsolete, so unlikely to be used still).
• Format the remaining build-tags for current go versions.

make tests linux-only

These tests were testing functionality that was only supported on
Linux, but stubbed for other platforms, so move them to a linux-only
file.

move package description to a doc.go

It's common practice to use a doc.go - in this case it helps with moving more code to platform-specific files, while still keeping a general description.

fix build-tags for non-linux platforms

• NSHandle could only be used on Unix systems. Given that all other parts
  of the code are only implemented on Linux (not for other Unix-y platforms),
  I moved this file to be Linux-only.
• Rename "_unspecified" to "_others", which is a common suffix for such cases.
• Introduce stubs for NSHandle for non-Linux platforms.

fix unhandled error in TestGetNewSetDelete

Just a minor nit / omission in the test

make deprecated consts an alias, and don't use internally

The consts were deprecated in favor of their counterparts in
golang.org/x/sys/unix. This patch makes them an alias / sets them
to those values, which makes it more transparent that they're the
same.

Also update internal uses of the deprecated consts, as they
should no longer be used, and updated the "stub" function
to be deprecated as well.

don't deprecate bindMountPath

The bindMountPath const was in the same group as the deprecated consts,
which were deprecated as a whole. This patch moves the bindMountPath
const outside of the group to make sure it's not considered deprecated.

GetFromName: use /run/netns instead of /var/run/netns

The GetFromName() function looked for network namespaces in "/var/run",
whereas DeleteNamed() and NewNamed() used the bindMountPath const (which
points to "/run/netns"). While "/var/run" should be symlink to "/run" on
most distros, this is not a guarantee, so use the same paths so that at
least the code is consistently using this path.

[thaJeztah]

@jeffwidman @dims @pacoxu @aboch PTAL

[jeffwidman]

Normally I prefer separate PR's, but here it's all super straightforward and already split at the commit level.

Nice catches, thank you!