libfetch fix for HTTPS reads

[Jonathan Perkin <jperkin%joyent.com@localhost>]

For the 2016Q3 illumos release I enabled HTTPS by default for
pkg_install and pkgin.  Unfortunately during testing it became
apparent there is an issue with fetching over HTTPS, where for some
reason (I haven't yet fully diagnosed it) EOF isn't detected or
propagated back correctly when running pkgin in -p mode, for example:

  $ pkgin -p up
  reading local summary...
  processing local summary...
  download started.
  pkgin: failure during fetch of file: Operation timed out

The difference with -p is that the progress bar isn't printed, so
there could be some interaction with the signals generated by the
progress bar update that makes it work when not using -p.

Looking at FreeBSD they have a couple of fixes in fetch_read() since
we last synced, and by backporting them I no longer see the the fetch
failures:

  https://github.com/joyent/pkgsrc/commit/c862e20f

I'm going to include this in the 2016Q3 illumos builds, but would also
like to fix this in trunk, so extra eyes to check there are no
regressions would be appreciated.  I've tested this on SmartOS and
Darwin which both had the -p issue, as well as NetBSD which doesn't
seem to be affected.

Thanks,

-- 
Jonathan Perkin  -  Joyent, Inc.  -  www.joyent.com