authplain: Use rawurlencode/decode when saving/loading password hash
from users.auth.php

This allows authplain to support password hash formats which embed ':'
in the hash string. (for example, the supported 'mediawiki' hash
format takes the form :B:<salt>:<md5hash>:)

This should be safely backwards compatible as the only time
rawurldecode will change the input string is if it sees a % sign
followed by two hex values. None of the existing hash formats use % as
a delimiter, and with base64 or hexadecimal encoding the % will not
appear in a field's value.

It does mean that older dokuwiki installs will not necessarily be able
to read passwords from newer users.auth.php files, though.