fix location sanitizing with parser 'future'

With parser 'future', a '\\' counts as one backslash whereas with
parser 'current' it's two. Therefore with 'future' only one gets passed
to regsubst which in turn aborts with

 Error: regsubst(): Bad regular expression `\'

Probably the 'current' behaviour is a bug, as
<http://docs.puppetlabs.com/puppet/latest/reference/lang_datatypes.html#single-quoted-strings>
states:

 When a literal double backslash is intended, a quadruple backslash must
 be used.

Yet, fixing either parser will break code so test here if '\\' is two
characters or one. Clumsy, but works.