MINOR: global: define tainted flag

MINOR: global: define tainted flag

Add a global flag named 'tainted'. Its purpose is to report various
status about experimental features used for the current process
lifetime.

By default it is initialized to 0. It can be set/retrieve by a couple of
new functions mark_tainted()/get_tainted(). Once a flag is set, it
cannot be resetted.

Currently, no tainted status is implemented, it will be the subject of
the following commits.