Revamp the mthread library and update test59

Revamp the mthread library and update test59

Before, the 'main thread' of a process was never taken into account anywhere in
the library, causing mutexes not to work properly (and consequently, neither
did the condition variables). For example, if the 'main thread' (that is, the
thread which is started at the beginning of a process; not a spawned thread by
the library) would lock a mutex, it wasn't actually locked.