libc: pthread-alike entrypoint for components

[chelmuth]

Currently, libc components that spawn further entrypoints for handling Genode-native interaction crudely use Libc::pthread_create_from_thread, which is a private libc utility. Providing Libc::Entrypoint in libc/component.h and adapting vbox5/6 cleans up this situation and enables regular use of the feature in third-party components.

[chelmuth]

The following commits implement a prototype of the feature in the context of test/tiled_wm.

5bc1bd7 WIP libc: provide pthread-compatible Libc::Entrypoint (tiled_wm-local)
2db74c1 WIP tiled_wm: use Libc::Entrypoint