libpod/container_internal: Split locale at the first dot, etc.

We're going to feed this into Go's BCP 47 language parser.  Language
tags have the form [1]:

  language
  ["-" script]
  ["-" region]
  *("-" variant)
  *("-" extension)
  ["-" privateuse]

and locales have the form [2]:

  [language[_territory][.codeset][@modifier]]

The modifier is useful for collation, but Go's language-based API
[3] does not provide a way for us to supply it.  This code converts
our locale to a BCP 47 language by stripping the dot and later and
replacing the first underscore, if any, with a hyphen.  This will
avoid errors like [4]:

  WARN[0000] failed to parse language "en_US.UTF-8": language: tag is not well-formed

when feeding language.Parse(...).

[1]: https://tools.ietf.org/html/bcp47#section-2.1
[2]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_02
[3]: https://github.com/golang/go/issues/25340
[4]: https://github.com/containers/libpod/issues/2494

Signed-off-by: W. Trevor King <wking@tremily.us>

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