[lager] Fit parameters in write to the new sendv syntax.

Summary: Fit parameters and removed SYSLOG_IDENTIFIER entry. This will be added by ejournald.

Test Plan: Do a log with lager_journald_backend

Reviewers: tbreddin, hwinkel, skattelmann

Reviewed By: tbreddin

CC: druss, aschultz

Maniphest Tasks: T222, T226

Differential Revision: https://review.tpip.net/D1009