STORM-1215: restore INSTALL_PROPRIETARY in Release builds, remove incorrect use of LL_RELEASE_FOR_DOWNLOAD from ReleaseOS

The reason was to manually resolve a couple of merge problems, as upstream has had several changes since this was applied, and some of the changes overlapped.

missed else case

fixed crash on startup

Automated merge with bundle:E:\code\viewer-experience-merge+c:\users\richard\appdata\local\temp\thg.ia7ixc\http__hg.secondlife.com_viewer-development_csqen8.hg

EXP-749 FIX [REGRESSION] Voice status indicator shown in IM session instead of profile pic in Basic and Advanced modes

reviewed by Leyla

EXP-752 FIX -- When "View Display Names" is disabled, only my own profile loads, when I click others profiles

Avatar username is now looked up in the name cache when it is blank, which happens when display names are turned off apparently.

Reviewed by Fredrik.

... so the last occurance of the build prefix in the executable path
will be matched. This should avoid cuttung too much subdirs away if
someone checks out the source to a path already containing
"/build-darwin-" or "/build-linux-".

Also cleaned up some whitespace-around-operator issues.
While removing the extra variable seems to make the code more ugly, all it does is reveal the pre-existing ugliness that had been masked in the past.  Trying to find a fix, or even try to come to a real understanding of why this section of code is the way it is, is out of my scope for this task.

Patch taken from here: https://paste.lindenlab.com/show/6902/

Ported Vadim's STORM-1182 fix for Linux (3d30b2bc1369) to the
corresponding Mac file.