Merge branch 'maint'

Merge branch 'maint'

* maint:
  options: try to report as much errors as possible
  warns: don't tell AM_PROG_MKDIR_P is going to be removed
  refactor: fix few "inverted boolean" usages
  options: better name for an internal function
  options: more consistency in use of return statuses to report errors
  options: tiny simplification in dealing with erroneous opts
  options: consistently use return statuses to report errors
  options: re-enable some sanity checks
  THANKS: update Eric Blake's e-mail address
  NEWS: typofix

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>