sendmail/procmail

About
Instructions which actually worked for configuring sendmail to deliver through procmail are here. Note the following:
 * "dnl" at the end of a command is optional, if there are no comments following it (it apparently means "delete from here to the start of the next line")
 * The "FEATURE(`local_procmail',`/path/to/procmail')dnl" line must go before the existing "MAILER(`local')" line
 * Be sure to restart sendmail after applying the change. (If not using Webmin to do the editing, also be sure to rebuilt the sendmail.m4 file.)

Links

 * Invoking procmail from sendmail: this seems to be about how to forward mail to procmail on a shared server, rather than how to configure sendmail to filter all mail through procmail
 * PROCMAIL_MAILER_PATH parameter
 * MAILER parameter
 * features: see "local_procmail" parameter
 * FAQ: Version 8 issues seems to have some procmail examples