Automatic merge from submit-queue (batch tested with PRs 50208, 50259, 49702, 50267, 48986)

Relax restrictions on environment variable names.

Fixes #2707

The POSIX standard restricts environment variable names to uppercase letters, digits, and the underscore character in shell contexts only. For generic application usage, it is stated that all other characters shall be tolerated. (Reference [here](http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html), my prose reasoning [here](https://github.com/kubernetes/kubernetes/issues/2707#issuecomment-285309156).)

This change relaxes the rules to some degree. Namely, we stop requiring environment variable names to be strict `C_IDENTIFIERS` and start permitting lowercase, dot, and dash characters.

Public container images using environment variable names beyond the shell-only context can benefit from this relaxation. Elasticsearch is one popular example.