Add copyright statement to the source files in the top-level directory (except the
trivial dummy-shlib.cc).

By the way, I don't think this code really belongs in the top-level directory.

Strangely, C++11's new std::chrono::system_clock::now() (which I wanted
to use in a test case) calls clock_gettime() not through the function,
but with the syscall() interface. So add support for that too.

This is only causing confusion; change all callers to add '\n' explicitly
and drop the optional argument.