REGTESTS: log: test the log filtering options of a frontend

REGTESTS: log: test the log filtering options of a frontend

"option dontlognull", "option dontlog-normal" and "option logasap" had
no coverage, and "option log-separate-errors" was only used as a side
setting of an SSL test.

Each option gets its own frontend: a connection on which nothing is
sent must not be logged while the next real request is, a successful
request must not be logged while the one ending on a 503 is, the same
pair must both be logged but at different syslog levels, and the
message emitted early must carry the '+' prefix on the byte count and
on the total time.

Two syslog servers are needed since vtest only matches the messages
emitted at the level its server was declared with, which is precisely
what log-separate-errors changes.