From scottr@netbsd.org:

From scottr@netbsd.org:
After servicing an interrupt, check to see that there's not another
one pending before we clear the interrupt flag.  This avoids a condition
where the line appears to go catatonic (which is particularly easily
triggered by pppd).  From Bill Studenmund <wrstuden@loki.stanford.edu>.
XXX - we should probably log this if it gets excessive.