capnslog: set SYSLOG_IDENTIFIER for journal
capnslog: set SYSLOG_IDENTIFIER for journal

Fixes #40. I think it's reasonable to just set this by default rather
than needing to expose it to users.

References:
http://www.freedesktop.org/software/systemd/man/systemd.journal-fields.html#SYSLOG_FACILITY=
http://linux.die.net/man/3/program_invocation_short_name

dgonyeo/pkg,jonboulle/pkg,barakmich/pkg,sym3tri/pkg,jonboulle/pkg,coreos/pkg,coreos/pkg,dgonyeo/pkg,sym3tri/pkg,barakmich/pkg