Bug 242690

It's not entirely clear if the current implementation semantics are fully identical between Linux and Apple/BSD systems. The Apple/BSD pthread_main_np function has 3 potential return values, 1 if the thread is the initial thread, 0 if it is not or -1 if thread is not initialized/unknown, lets make sure we only return true for isMainThread if the return for pthread_main_np is 1. On linux checking if we are on the initial thread can be accomplished by checking the equality of getpid and gettid. Note we use a syscall for calling gettid as the gettid() libc function may not be guaranteed to be supported by all libc's on Linux unlike the syscall(which is also used elsewhere in WebKit).