Remove vget from global namespace

["J. Hannken-Illjes" <hannken%eis.cs.tu-bs.de@localhost>]

With the global vnode cache there is no longer a need for vget() (take
the first reference on a vnode) to reside in global namespace.
 
1) Rename vget() to vcache_vget() and vcache_tryvget() respectively and    
   move the definitions to sys/vnode_impl.h.
   
2) Vget() increments the v_usecount before it checks the vnode is stable.
   This behaviour makes it impossible to assert v_usecount being zero
   or one as v_usecount also reflects other threads trying to vget().
   
   Change vcache_vget() to increment v_holdcnt to prevent the vnode from 
   disappearing while vcache_vget() waits for a stable state and 
   free the vnode if this hold was the last reference to a reclaimed vnode.
   
3) Add some "v_usecount == 1" assertions.
 
Diffs are here: https://www.netbsd.org/~hannken/vget/

Comments or objections anyone?

--
J. Hannken-Illjes - hannken%eis.cs.tu-bs.de@localhost - TU Braunschweig (Germany)