Merge branch 'next'

* next: (37 commits)
  minor, migrate unsermon to new API, losing the capability to decode and display uterus messages
  chore, move ud-sockaddr.h inclusion in unserding.h to UD_COMPAT section
  cosmetics, leave a minor note about the longevity private API
  minor, provide UD_CHN() macro (converse of UD_SVC()) and express ctrl msg predicate in terms of that
  minor, rewrite ud_chck_sa() as ud_chck_aux() with more slots
  fix, PEBKAC, initialise the sz slot of the src sa to the size of ud_sockaddr_u
  minor, hand out receiver data through (experimental) ud_chck_sa()
  cosmetics, talk about libev even in comments, in SXE_CHECK_LIBEV (was expat left-overs)
  fix, PEBKAC, properly quote cases in PKG_CHECK_MODULES_HEADERS()
  minor, start migration of unsermon (away from UD_COMPAT)
  cosmetics, update file headers of unsermon.c
  minor, migrate unsermon to ud-logger facility
  hygiene, include syslog.h again in ud-logger.h so facilities are defined
  minor, migrate mcast.c to ud-logger system and define UD_SYSLOG convenience macro
  minor, make ud_logout() the most generic function and plaster error() and logger() on top
  chore, adapt unserding-dbg.h to new logger system
  minor, rename logger.h to ud-logger.h
  minor, in ud-ping check if response packet is actually a PONG
  minor, track PING or PONG in ud_chck_ping()
  minor, deflect control messages in ud_chck_msg() to ud_chck_cmsg()
  ...