Begin updating strings in HTTP setup.

This patch is the first in a series of patches that will update the
strings in backend HTTP setup to agree with the changes we have made to
the strings in programs/mythtv-setup/backendsettings.cpp since the
original XML files were created (over a year before the XML files were
added to master).  This should make more of the translations work (and
prevent translators from having to re-tranlate strings that were
supposed to have been removed and replaced with better strings).

I have made every attempt to not revert changes that were made only in
the XML files with the intention of clarifying the strings (and may
eventually backport them to mythtv-setup's backend settings just so we
have a single string for translators).  Robert K has independently made
a couple of the changes that were already made to mythtv-setup's
strings, but this series (when complete) should prevent further
duplication of efforts.

I've only checked settings up through UPNP Server Settings in
config_backend_general.xml, but wanted to get this in while it still
applies, since I likely won't work on this further until next week.

Changes incorporated as part of this series (will) include those made in
7c46b75 ,
75dc7cf,
24e854a,
fe1c8bf,
fcfe996 , and possibly others

In addition, this patch:
 - Adds StorageScheduler option BalancedPercFreeSpace (from
   b9efe92 )
 - Removes EITIgnoresSource setting (from
   c2c7de4 )