Refactor crypttab(5) parsing logic.

Going through each of the crypttab(5) file, ignoring empty lines and
comments, decoding octal escape sequences, etc. introduced a lot of
duplicate boilerplate code.  We now use

    crypttab_find_entry([--quiet], $target)

        Search the crypttab(5) for the given $target and set
        CRYPTTAB_NAME, CRYPTTAB_SOURCE, CRYPTTAB_KEY, and
        CRYPTTAB_OPTIONS accordingly.  Return 0 if a match is found, and
        1 otherwise.

and

    crypttab_foreach_entry($callback)

        Iterate through the crypttab(5) and run the given $callback for
        each entry found.  $callback refers to each entry using
        variables CRYPTTAB_NAME, CRYPTTAB_SOURCE, CRYPTTAB_KEY and
        CRYPTTAB_OPTIONS.