Add property-based filters for syslogd.

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
blackslash.

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>
MFC after:	2 weeks
Relnotes:	yes
Differential Revision:	https://reviews.freebsd.org/D23468