Fix bugs in OSPF MD5 authentication. First bug is that default

Fix bugs in OSPF MD5 authentication. First bug is that default
values for MD5 password ID changed during reconfigure, Second
bug is that BIRD chooses password in first-fit manner, but RFC
says that it should use the one with the latest generate-from.

It also modifies the syntax for multiple passwords.
Now it is possible to just add more 'password' statements
to the interface section and it is not needed to use
'passwords' section. Old syntax can be used too.