#14 Add support for insheader()

[Todd Vierling]

Detection of this capability is tricky -- in the
default Sendmail 8.13 install, it's available, but the
SMFI_VERSION is still 2. (We can assume that version 3
or 4 has it available.)

It will probably be necessary to put in a stern
programmers' warning about using such a method if the
server is not Sendmail 8.13 or later.