Improved file buffering in CArchive.

CArchive already did some file buffering, but only on writes. Added the
equivalent code for reads. Also improved the write buffer case so that it
only ever issues sector-aligned writes (the read code does this from the
start). Shuffled various bits of code into the header file to squeeze a bit
more performance out of it.
[application] - remove m_bInBackground and its setter/getter and use
SetRenderGUI instead
[airtunes] - phase out support for libshairport - its superseeded by libshairplay long enough on all platforms
Correct null check

Correct likely copy and paste mistake, and null check the correct
variable.

Fixes Coverity CID 1213840
Merge pull request #4234 from bavison/carchive_buffering

Improved file buffering in CArchive.
Merge pull request #4697 from Memphiz/phaseoutshairport

[airtunes] - phase out support for libshairport - its superseeded by lib...
Merge pull request #4693 from Memphiz/remove_background

[application] - remove m_bInBackground and its setter/getter and use
Merge pull request #4701 from cg110/CoverityFixes4

Fixes for the obvious bug from this weeks coverity scan.
Merge remote-tracking branch 'upstream/master'