RAdv: Add custom options

Currently one can use only a predefined set of advertised options in RAdv
protocol, which are supported by BIRD configuration. It would be convenient
to be able to specify other possible options at least manually as a blob
so one should not wait until it is supported in the code, released, etc.

This idea is inspired by presentation by Ondřej Caletka at CSNOG, in which
he noticed the lack of either PREF64 option or possibility to add custom
options in various software.

The patch makes it possible to define such options with the syntax:

  other type <num> <bytestring>

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