log: Introduce PULSE_LOG_JOURNAL environment variable

log: Introduce PULSE_LOG_JOURNAL environment variable

By introducing such an environment variable, applications using the
PA client libraries can configure these libraries to send their logs
directly to the journal.

While client libraries journal logging can be indirectly achieved
using PULSE_LOG_SYSLOG, this pollutes the journal. Meta data gets
replicated twice: once in the journal meta fields and once in the
syslog(3) plain-text message itself.

For attaching any backtraces, also introduce the PA-specific journal
meta field PULSE_BACKTRACE. This is the recommend journal practice
instead of appending any furuther data to the logging message itself.

Signed-off-by: Ahmed S. Darwish <darwish.07@gmail.com>