Cleanup the textvp_fullpath() function; summary of changes:

Cleanup the textvp_fullpath() function; summary of changes:

	* rename it to vn_fullpath(), and move prototype to sys/vnode.h;
	  for now, keep a compat macro called textvp_fullpath(); this
	  macro is temporary until I change linprocfs and procfs to play
	  with vn_fullpath().

	* rename last argument to 'freebuf', instead of 'retfreebuf'.

	* if the second argument (vnode pointer) is NULL, fall back
	  and use the passed-in process' p->p_textvp.

	* make sure the client passes a process pointer, otherwise
	  return EINVAL.

A manual page will be added real soon.