Optimize vm_page_try_to_free(). Specifically, the call to pmap_remove_all()
can be avoided when the page's containing object has a reference count of
zero. (If the object has a reference count of zero, then none of its pages
can possibly be mapped.)

Address nearby style issues in vm_page_try_to_free(), and change its
return type to "bool".

Reviewed by: kib, markj
MFC after: 1 week