vacuumlo nonstandard use of \\ in a string literal

In 8.1 and HEAD, if a role has escape_string_warning enabled then
vacuumlo elicits the following warning:

WARNING:  nonstandard use of \\ in a string literal at character 323
HINT:  Use the escape string syntax for backslashes, e.g., E'\\'.

How backward compatible should vacuumlo remain?  This patch changes
the string to E'...' but other ways would be to use dollar quotes
or ESCAPE.

--
Michael Fuhr