functrace traces the function calls a program makes.
It's like strace, but for userspace.
It works by using Debugging functionality to insert breakpoints
at call&ret instructions.
Previously, a breakpoint was removed after it was tripped.
After a breakpoint trips, we have to undo the 'int3' patch
from the instruction in order to continue the exceution.
To make a breakpoint persist, we switch to "single step" mode,
which stops the execution after a single instruction, and then we
insert the breakpoint at the previous instruction.
There is also some code that deals with an edge case where there are
breakpoints in two consecutive instructions.
