(PUP-1814) Make double backslash in sq string behave as documented.

This makes it possible to escapce a backslash in a sq string. 
Previously this did not work which made it impossible to end a sq string
with a backslash.
Merge pull request #2400 from hlindberg/pup-1814_double-backslash-in-sqstring

(PUP-1814) Make double backslash in sq string behave as documented.