Irritating how many different places we have to touch to add a library...

This addresses a Linux link failure due to the Linux linker making a single
left-to-right pass through libraries.

In autobuild.xml, specify today's build of the Boost package that includes the
Boost.Context library, and whose boost::dcoroutines library uses Boost.Context
exclusively instead of its previous context-switching underpinnings (source of
the ucontext.h dependency).
Add BOOST_CONTEXT_LIBRARY to Boost.cmake and Copy3rdPartyLibs.cmake. Link it
with the viewer and with the lllogin.cpp test executable.
Track new Boost package convention that our (early, unofficial) Boost.Coroutine
library is now accessed as boost/dcoroutine/etc.h and boost::dcoroutines::etc.
Remove #include <boost/coroutine/coroutine.hpp> from
llviewerprecompiledheaders.h and lllogin.cpp: old rule that Boost.Coroutine
header must be #included before anything else that might use ucontext.h is
gone now that we no longer depend on ucontext.h. In fact remove
-D_XOPEN_SOURCE in 00-Common.cmake because that was inserted specifically to
work around a known problem with the ucontext.h facilities.

- Removed duplicate entry from panel_preferences_general.xml

- Removed duplicate notification elements

* A fix that doesn't break tanks. Reverts MAINT-1742

* A fix that doesn't break doors. Reverst MAINT-2247

* Removed bogus check on matching parcel data coming in.