Added custom recursive stripslashes function

The previous use of `PMA_arrayWalkRecursive` with `stripslashes` required multiple
passes through the array if keys/values had more than 1 slash. To make it more
performant, I added a recursive `stripslashes` function `PMA_stripAllSlashes`
that strips all slashes from keys/values on the first pass.

Signed-off-by: Jeff Chan <jefftchan@gmail.com>