This adds a parameter 'milters' to the sendmail::configure defintion,
which should be a hash specifying what mail input filters Sendmail
should apply.  Role classes are then passed a new 'milters' parameter,
and similarly to the 'localdomains' and 'defines' parameters, it will
receive a simple string with the proper INPUT_MAIL_FILTER() calls.

Role classes and their ERB templates are updated to handle the new
parameter, and insert it into the generated M4 files.