Merge branch 'dk/parseopt-optional-filename-fixes' into next

A recently added configuration variable and command line option
syntax ":(optional)" for values that are of filename type
inconsistently behaved on an empty file (configuration took it
happily, while the command line option pretended as if it did not
exist), which has been corrected.

* dk/parseopt-optional-filename-fixes:
  parseopt: remove unreachable code
  parseopt: restore const qualifier to parsed filename
  config: use boolean type for a simple flag
  parseopt: use boolean type for a simple flag
  doc: clarify command equivalence comment
  parseopt: fix :(optional) at command line to only ignore missing files

diff --cc config.c
Simple merge

diff --cc parse-options.c
Simple merge