Tweaked the siointr1() so that it works better at 921600 bps, especially

with multiple ports on a shared interrupt demultiplexed by the puc_intr()
handler.

siointr1() first read as much input as possible and then checked all
possibly-relevant status registers, partly for robustness and partly
for historical reasons.  This is very bad if it is called for every
port sharing an interrupt like puc_intr() does.  It can spend too long
reading all the input for some ports when the interrupt is for a more
urgent event on another, or just too long checking all the status
registers when there are lots of ports.  The inter-character time is
too long for reading all the input even when the interrupt is for a
transmitter interrupt on the same port, and at 921600 bps the inter-char
time is 10.85 usec and was often exceeded with just 2 ports, leaving
the transmitters idle for about 6% of the time.

The tweak is to break out of the read loop after reading 1 char if
output can be done.  This avoids most of the idle transmitter time for
2 active ports at 921600 bps bidirectional on the test system.  It
also reduces overhead by about 20%.  More complete fixes use the
programmable tx low watermark on 16950's and reduce overhead by another
65%.

sys/dev/sio/sio.c

@@ -1868,6 +1868,10 @@ if (com->iptr - com->ibuf == 8)
 					CE_RECORD(com, CE_OVERRUN);
 			}
 cont:
+			if (line_status & LSR_TXRDY
+			    && com->state >= (CS_BUSY | CS_TTGO | CS_ODEVREADY))
+				goto txrdy;
+
 			/*
 			 * "& 0x7F" is to avoid the gcc-1.40 generating a slow
 			 * jump from the top of the loop to here
@@ -1905,6 +1909,7 @@ if (com->iptr - com->ibuf == 8)
 			}
 		}
 
+txrdy:
 		/* output queued and everything ready? */
 		if (line_status & LSR_TXRDY
 		    && com->state >= (CS_BUSY | CS_TTGO | CS_ODEVREADY)) {