cmogstored 1.7.0

The big feature in this release is "devNNN/usage" are served
from memory, allowing up-to-date usage information even
unwritable/unreadable filesystems.

This can also be used to reduce spinups and wear on HDDs.

"devNNN/usage" files are still updated on the FS by default for
compatibility with existing HTTP servers, but admins may wish
to disable updates to them by removing all permissions from
the "usage" files:

	chmod 0000 $MOG_DOCROOT/dev*/usage

Filesystem errors from the sendfile(2) syscalls are also
logged to syslog.  There's also a bugfix for zombies for
libkqueue-on-epoll users, but that doesn't affect native
kqueue users on *BSDs.

And the usual round of gnulib, minor doc and style updates.

18 changes since v1.6.0:

      cmogstored.h: remove unused mog_file.mmptr member
      doc: documentation for ioq
      doc: further comment updates around ioq
      build-aux/txt2pre: support '=' in URLs
      test/inherit: fix ambiguous parenthese warning
      test/inherit: stop testing Ruby itself
      doc: update URLs to HTTPS
      compat_sendfile: ensure this works without an offset
      doc/queues.txt: add key point about only retrieving ONE event
      fix trace.h dependency on probes.h
      update to gnulib.git 90f289f249a266b1afb9c63e182f5d979d17df5f
      http_get.c: log filesystem-level errors from sendfile
      serve /dev*/usage requests from memory
      doc: URL updates to reduce redirects and favor HTTPS
      test/inherit.rb: fix syntax error under Ruby 1.8
      update copyrights for 2018 and use SPDX for "GPL-3.0+"
      selfwake: enable self-pipe with kqueue
      http_parser: workaround parsing OOM in Ragel 6.10