Introduce `rtlabel_mtx' mutex(9) to protect route labels storage. This

Introduce `rtlabel_mtx' mutex(9) to protect route labels storage. This
time kernel and net locks are held in various combination to protect it.
We don't want to put kernel lock to all the places. Netlock also can't
be used  because rtfree(9) which calls rtlabel_unref() has unknown
netlock state within.

This new `rtlabel_mtx' mutex(9) protects `rt_labels' list and `label'
entry dereference. Since we don't export 'rt_label' structure, keep this
lock private to net/route.c. For this reason rtlabel_id2name() now
copies label string to externally passed buffer instead of returning
address of `rt_labels' list data. This is the way which rtlabel_id2sa()
already works.

ok bluhm@