MFC r359327,359328:

MFC r359327,359328:
  Add property-based filters for syslogd.

  Property-based filters allow substring and regular expressions
  (see re_format(7)) matching against various message attributes.
  Filter specification starts with '#:' or ':' followed by three
  comma-separated fields property, operator, "value". Value must be
  double-quoted. A double quote and backslash must be escaped by a
  backslash.

  Following properties are supported as test value:
  o msg - body of the message received;
  o programname - program name sent the message;
  o hostname - hostname of message's originator;
  o source - an alias for hostname.

  Supported operators:
  o contains - true if filter value is found as a substring of property;
  o isequal - true if filter value is equal to property;
  o startswith - true if property starts with filter value;
  o regex - true if property matches basic regular expression defined
      in filter value;
  o ereregex - true if property matches extended regular expression
      defined in filter value;

  Operator may be prefixed by '!' to invert compare logic or by
  'icase_' to make comparison function case insensitive.

  Submitted by:	Boris N. Lytochkin <lytboris at gmail com>
  Relnotes:	yes
  Differential Revision:	https://reviews.freebsd.org/D23468