[ng] vars: names for internal make variables: 'am.foo' and 'am.foo.bar-baz'

[ng] vars: names for internal make variables: 'am.foo' and 'am.foo.bar-baz'

That is the new preferred naming scheme: 'am.foo' where we would
have previously used something like 'am__foo', and 'am.foo.bar-baz'
where we would have previously used something like 'am__foo_bar_baz'
or 'am__foo__bar_baz'.

We should start using the new naming to do so in future commits.  But
we should also avid a sweeping rename for now, to minimize conflicts
with the mainline Automake codebases, which (for portability reason)
must still limit itself to the use of the 'am__' prefix.

* HACKING: Adjust.  Also, remove now-irrelevant advice about the
problem of an old vendor make (NEWS-OS 4.2R) with variables whose
name start with an underscore.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>