builtin: Keep backslash on quotes outside of dollarsq

Only dollar single quote should eat the backslash character before
a quote.  Make the skipping of the backslash conditional on mbchar
in conv_escape.

Reported-by: Juergen Daubert <j.daubert@posteo.de>
Fixes: 776424a8f915 ("parser: Add dollar single quote")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>