authplain: Escape ':' in any data field as '\:'

':' is the field delimiter in the authplain flat text
conf/users.auth.php file, but it's also used as an internal delimiter
for the 'mediawiki' password hash format. Currently using this hash
format corrupts the file

This change escapes ':' as '\:' in any field in the users.auth.php
file, and any '\' as '\\'.

Also adds test cases for escaping modes.