vm_pager.h
Added argument to getpbuf() and relpbuf() to allow each subsystem to
specify a different hard limit on the number of simultanious physical
bufferes that said subsystem may allocate. Without this feature, one
subsystem ( e.g. the vfs clustering code ) could hog *ALL* the pbufs,
causing a deadlock in the pager in a low memory situation.
Same for trypbuf().