config(8): Make 'env' files consistent with other file-accepting options

Previously, only one 'env' file could be specified. Later 'env' directives
would overwrite earlier 'env' directives. This is inconsistent with every
other file-accepting directives which process files in order, including
hints.

A caveat applies to both hints and env that isn't mentioned: they're
concatenated in the order of appearance, so they're not actually applied in
the way one might think by supplying:

hints x
hints y

Hints in x will take precedence over same-name hints in y due to how
the kernel processes them, stopping at the first line that matches the hint
we're searching for. Future work will flip the order of concatenation so
that later files may still properly override earlier files.

In practice, this likely doesn't matter at all due to the nature of the
beast.