Revert "VirtualGL as default, primus as alternative"

This reverts commit a22b602ebb345a9bba4684970a3487ffd2a45b45.

Since this branch will only be merged when primus will be ready for replacing VirtualGL as default, there is no need for this.
Merge branch 'common-primus' into develop
Add release notes for 3.1
README: add primus, safer gpasswd command
Updated RELEASE_NOTES_3_1

Be more descriptive about primus.

Changed date for real release date.
Merge branch 'release-3.1' of github.com:Bumblebee-Project/Bumblebee into release-3.1
Removed trailing spaces in release notes, typo fix
Merge branch 'release-3.1'
Merge branch 'release-3.1' into develop
Merge branch 'master' into develop
Rename confusing stop_on_exit
Revert "Rename confusing stop_on_exit"

This reverts commit 1de850e5863d28fed508cbfff5ded13114574e58.

The name is actually correct, but the debug message is misleading.
Add /usr/$LIB/mesa/libGL.so.1 to PRIMUS_libGLd

Ubuntu 13.04 uses `lib/x86_64-linux-gnu` for `$LIB`. Reported in GH-334.
Fix documentation of --pidfile option

Add missing "FILE" and while I am at it, extend the description.
bumblebeed: Fix read past buffer (GH-355)
optirun: fix read past buffer on illegal daemon response
Add missing declaration for ensureZeroTerminated

Fixes: src/bumblebeed.c:171:5: warning: implicit declaration of function
‘ensureZeroTerminated’ [-Wimplicit-function-declaration]

The missing declaration did not affect the fixes made in da77e37 and c0babc1 (at
least for gcc) as bbsocket.o is already included.
add option to set the primus ldpath, and use it when using the primus bridge - primusrun is no longer needed
add PrimusLibraryPath to bumblebee.conf
add bash completion for --primus-ldpath