Registers a trace variable for a boolean reference.
LIBRARY
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DESCRIPTION
A call to register_btrace(nm, bref) registers a trace variable
called nm that can take on two different values (0 and 1), which
correspond to the state of the boolean variable bref.
FAILURE
Fails if the given name is already in use as a trace variable.
COMMENTS
This function uses register_ftrace to make a boolean variable appear
as an integer value.