regen/warnings.pl: add magic/env_ref_value warning class

Increment version number

And regenerate:

	lib/warnings.pm
	warnings.h
mg.c: Raise a warning when a reference is inserted in %ENV

This warning happens prior to stringification.

Avoid the % character in output
pod/perldiag.pod: Explain warning given by WARN_ENV_REF_VALUE

Use only the warning name, not its group

Wording revised as per Jim Keenans suggestions
t/op/magic.t: test for the warning with %ENV set, and suppress the warning from test output