1. libcobalt now controls dynamically whether auto-shadowing of the
current context is disabled by checking the new XENO_NOSHADOW
environment variable for existence. Otherwise, the current thread
becomes a Xenomai thread when the library constructor is executed.

This replaces the former static mechanism based on building with
--enable-dlopen-skins, aimed at dynamic loading of the POSIX interface
via dlopen(). Applications can run putenv("XENO_NOSHADOW=<non-empty>")
before dynamically loading libcobalt.so.

2. global memory locking is systematically performed from the library
constructor if auto-shadowing is enabled, and remains in effect
afterwards. Applications which do not want such locking to be in
effect during their initialization phase should call munlockall()
explicitly, then lock the process memory anew before creating any
Xenomai thread.

3. The new --enable-dlopen-libs build switch must be passed to allow
Xenomai libaries to be dynamically loaded via dlopen(). This replaces
the former --enable-dlopen-skins switch. Unlike the latter,
--enable-dlopen-libs does not implicitly disable TLS support.

3. The --with-__thread build switch has been renamed --enable-tls. TLS
is now available to dynamically loaded Xenomai libraries,
i.e. --enable-tls --enable-dlopen-libs is valid.