[syslog-ng] Default name value pair

[John Kristoff]

Perhaps the functionality is there and I don't realize it, and this is
not critical if it's not, but it would come in handy to be able to do
something like the following:

 destination d_sql {
   sql(
       ...
       values( "${FOO:-default_value}", ...)

In other words, being able to specify a default value for the
name-value pair when one doesn't exist much like you can do in UNIX
shell scripts.

Similarly, but in my case otherwise, using patterndb, it would be handy
to be able to set a name-value pair per <pattern>.  I believe right now
you can only do do based on the upper level <patterns> hierarchy.

To help clarify the reasoning, I have a database table with a field
that I'd like to fill with at least a default value, but that field
might not exist in the log message in some cases.  For instance, BIND
9.x query logs can log the "view", but unless you configure views for
all the possible classes, which is generally not done, queries for
other classes will not have the view identifier.  Compare these common
examples:

  <test_message>queries: info: client 127.0.0.1#1024: view lopback: query: 1.0.0.127.in-addr.arpa IN PTR +</test_message>
  <test_message>queries: info: client 192.0.2.1#1024: query: version.bind CH TXT + (192.0.2.1)</test_message>

Right now the view field gets logged as a null, which is OK, but I'd
rather it be inserted with a default view of 'default' in this example.

John