minor, provide dflt_tm() getter/setter and use that in the massaging routine

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
minor, provide api function dt_set_default()

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
minor, support --default=DT in dconv tool, using dt_set_default() and the new massaging routine

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'feat/default' into next

* feat/default:
  minor, support --default=DT in dconv tool, using dt_set_default() and the new massaging routine
  minor, provide api function dt_set_default()
  minor, provide dflt_tm() getter/setter and use that in the massaging routine
Merge branch 'next'

* next:
  minor, support --default=DT in dconv tool, using dt_set_default() and the new massaging routine
  minor, provide api function dt_set_default()
  minor, provide dflt_tm() getter/setter and use that in the massaging routine
  minor, go straight to time massaging in massage_strpdt() when no strpd_s field is set
  Revert "minor, leave a note about our massaging business in strpdt_s objects"
  minor, leave a note about our massaging business in strpdt_s objects
  minor, don't check (or set) nanosecond field, nothing is below that in the hierarchy
  minor, instead of checking strpt_s values for 0, check their set field (h_set, m_set, ...)
  minor, split up strpt_s' component_set field into h_set, m_set, etc.
  hygiene, follow strpt_s' union wrap of flags field in strpd_s
  hygiene, wrap strpt_s flags in union named flags
  minor, provide a massage routine for strpdt_s objects filling in missing values according to global `now'
  hygiene, provide and use singletons (now_tv() and now_tm()) to obtain the current date/time, ...
  chore, provide handy helper with(){} in nifty.h
  hygiene, move comments where they belong
  hygiene, don't expose __trans_dtfmt()
  hygiene, don't expose __trans_dfmt()