Merge branch 'refs/heads/t/duration-keyword'

A single patch which was created already in 2018 but did not propagate
to next and master for no good reason.

Cooking for a month.

* refs/heads/t/duration-keyword:
  mood: Make "duration" a new keyword for the mood grammar.

diff --combined NEWS.md
index 073f26471a101310cc92e484f4c410aa64a6f9d2,3e3e7abfda28365cba7abfa2ead1c2c408d75801..1082671002acf196e004abae216f8b6dd1957c65
--- 1/NEWS.md
--- 2/NEWS.md
+++ b/NEWS.md
@@@ -1,56 -1,6 +1,58 @@@
  NEWS
  ====
  
 +---------------------------------------
 +0.7.0 (to be announced) "seismic orbit"
 +---------------------------------------
 +
++- The new "duration" keyword of the mood grammar makes it possible to
++  impose a constraint on the duration of the admissible files.
 +- Paraslash writers handle early end-of-file more gracefully.
 +- The alsa writer no longer warns about spurious underruns.
 +- The score formula  of the audio file selector has been reworked.
 +- Cleanups of the doubly linked lists code.
 +
 +--------------------------------------
 +0.6.4 (2021-11-04) "fuzzy calibration"
 +--------------------------------------
 +
 +This point release contains a fair number of fixes but no new features.
 +This marks the end of the 0.6 development, although paraslash-0.6 will
 +still be supported for some time and subsequent maintenance releases
 +may follow.
 +
 +- The udp sender no longer crashes when empty chunks are encountered.
 +- Fix a double-free bug in the exit path of the server.
 +- The "jmp" command now errors out when given a negative percentage.
 +- A fix for a bug in para_afh which triggered on the attempt to modify
 +  the tags of an invalid mp4 file.
 +- A memory leak in para_afh has been fixed.
 +- The udp sender no longer sends multiple EOF packets.
 +- Some gcc warnings have been silenced.
 +- Minor log level adjustments and documentation improvements.
 +
 +Downloads:
 +[tarball](./releases/paraslash-0.6.4.tar.xz),
 +[signature](./releases/paraslash-0.6.4.tar.xz.asc)
 +
 +---------------------------------------
 +0.5.9 (2021-11-04) "reversed dimension"
 +---------------------------------------
 +
 +This release contains a few important fixes which have accumulated in
 +the maint branch. The paraslash-0.5.x series has now reached its end
 +of life and will no longer be supported. All users should upgrade to
 +a more recent version at this point.
 +
 +- Fix an issue with the bash completion script.
 +- Initialize the random seed also when using libgrypt.
 +- Fix some compiler warnings in the resample filter
 +- Don't return spurious errors from the ff server command.
 +
 +Downloads:
 +[tarball](./releases/paraslash-0.5.9.tar.bz2),
 +[signature](./releases/paraslash-0.5.9.tar.bz2.asc)
 +
  ----------------------------------------
  0.6.3 (2021-02-18) "generalized activity"
  -----------------------------------------