Factor out code to read mailboxes, and make read more robust

Factor out code to read mailboxes, and make read more robust

New tel_read_header(), tel_read_body().  Use them in rea(),
show_first_tel(), copy_and_expire().

rea() now stops when it encounters a corrupt telegram, and logs the
problem.  Before, error detection was incomplete, and errors were not
logged.  Corrupt mailboxes could make it crash.

show_first_tel() and copy_and_expire() can now cope with telegrams of
arbitrary length, like rea(), and sanity-check the header fields they
don't actually use.