cmd/acme: Dump creates corrupted acme.dump files

[rogpeppe]

I'm aware that cmd/acme isn't under active maintenance, but filing this here anyway,
because I've just started to use it in earnest after one too many memory corruption issue
with the original acme and it seems like a reasonable place to record issues.

Here's a portion of a corrupted dump file, created by middle-clicking on Dump:

f          2           2           0           0   2.0467836 
          2           6           0           0           0  Look 
e          2           0           0           0   4.0935673 
          6          75       14595           0           1 /home/rogpeppe/other/plan9port/src/cmd/acme/-snazzy2 Del Snarf | Look  Send
/home/rogpeppe/
win
e          2           0           0           0   6.1403509 
         21          58       25466           0           1 /home/rogpeppe/other/9fans/-snazzy2 Del Snarf | Look  Send
/home/rogpeppe/other/9fans/cmd/acme/internal/wind/
win
F          2           3       14274       14280  69.3957115       33847 
         10          86       33847           0           0 /home/rogpeppe/other/plan9port/src/cmd/acme/text.c Del Snarf Redo | Look notdef exec.c

This causes acme to panic on Load because there's a header line with no | character.