rpki-client: remove the remaining struct parse

rpki-client: remove the remaining struct parse

With the exception of mft.c where there is an additional boolean, this
struct carries a file name and a result. This means functions having
struct parse in the signature can't be shared between files, which has
been annoying. Simply pass file name and necessary info directly as a
function parameter and add a small dance to handle the boolean in mft.c.

ok job