(#20229) Fix per-interface netmask_* facts with net-tools 1.60

Without this patch the interface specific facts for the netmask are not
correctly parsed on systems with net-tools 1.60 or later, such as
Archlinux.

This patch addresses the problem by adjusting the regular expression to
take into account the new format of the ifconfig command.

This patch is specific to the netmask_* facts and does not affect the
behavior of the general `netmask` fact.
(#20229) Fix netmask fact with Linux net-tools 1.60

Without this patch the netmask fact does not return a value on Archlinux
because of the new format of the net-tools package.

This patch addresses the problem by updating the regular expressions for
the netmask fact.  Both the old format and the new format are covered by
the regular expression now.
(maint) De-duplicate parses RSpec alias

Adrien pointed out the duplication of

    RSpec.configure do |config|
      config.alias_it_should_behave_like_to :example_behavior_for, "parses"
    end

in both of the interfaces_spec file and the netmask_spec file.  This
patch removes the duplicate statements and places them in a shared file
required by both example specs.