lib: parser: remove startnode & sentence_root

lib: parser: remove startnode & sentence_root

This removes an artificial restriction for the first token in a
command's graph to be a WORD_TKN.  The intention seems to be to prohibit
empty paths through a command, and to restrict "explosion" of choices in
the root node.

The better approach to the former is to check for an empty path after
the definition is parsed.  The latter will happen anyway, by duplication
of the command, which just makes it worse...

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>