Unlocking a mutex can never throw an exception. That doesn't make
sense and our code should make sure it can't happen. Right now,
provided that the system-level functions don't throw, we don't either.

Locking a mutex cannot throw on Linux because we use futexes
directly. A non-recursive mutex is just a futex, whereas a recursive
mutex uses a mutex (a futex) to manage a lock count.

However, on other platforms, due to the freelist, there can be memory
allocation, which means it might throw std::bad_alloc. Not because of
the freelist itself (that uses malloc and will just crash if malloc
fails) but because of Q_GLOBAL_STATIC. In 5.1, the global static will
be noexcept provided the type's constructor is so too (it is, in this
case).

Change-Id: I4c562383f48de1be7827b9afb512d73eaf0792d5
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>