lib: add namespace name structure in zebra message

lib: add namespace name structure in zebra message

The addition of the name of the netns in the vrf message introduces also
a limitation when the size of the netns is bigger than 15 bytes. Then
the netns are ignored by the library.
In addition to this, some sanity checks have been introduced. some
functions to create the netns from a call not coming from the vty is
being added with traces.
Also, the ns vty function is reentrant, if the context is already
created.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>
Signed-off-by: Renato Westphal <renato@opensourcerouting.org>