usernames with trailing underscores should lose less loudly

[kaduk]

Upon login, from(1) and mailquota(1) both error out with, if I remember correctly, "Cannot find Post Office server for $username".
This seems to be due to the use of gethostbyname($username.mail.mit.edu), which has trouble due to the effect of trailing underscores on the DNS.

We should check for this and either fall back to hesiod or exit silently, instead of erroring out in the normal case when everything is functioning as expected.