Fix a number of issues and bugs with regsub():
Fix a number of issues and bugs with regsub():

Allow '\' to escape the next char.

Make '$$' be the same as '&': entire match
XXX: NB: Should we drop the sed(1) usage of '&' here, it means
XXX: a lot of backslashing in query string urls ?

Make $-digit work correctly.

Fixes ticket 249



git-svn-id: 7d4b18ab7d176792635d6a7a77dd8cbbea8e8daa@2739 d4fa192b-c00b-0410-8231-f00ffab90ce4

wikimedia/operations-debs-varnish,wikimedia/operations-debs-varnish,CartoDB/Varnish-Cache,ssm/pkg-varnish,ssm/pkg-varnish,ssm/pkg-varnish,CartoDB/Varnish-Cache,wikimedia/operations-debs-varnish,ssm/pkg-varnish,wikimedia/operations-debs-varnish,CartoDB/Varnish-Cache,ssm/pkg-varnish,wikimedia/operations-debs-varnish