Correct gecos expansion when From: is a prefix of the username.

Test 0254 submits a message to Exim with the header

  Resent-From: f

When I ran the test suite under the user fanf2, Exim expanded
the header to contain my full name, whereas it should have added
a Resent-Sender: header. It erroneously treats any prefix of the
username as equal to the username.

This change corrects that bug.