Fix OpenBSD codepath, plus some code cleanups

List of changes:

• Fix OpenBSD codepath
• Add xfce4::Optional utility type
• Move parse_ulong() to xfce4 namespace
  • parse_ulong() isn't OS-specific, so it shouldn't be in the os.cc file
• Let Linux read_topology() compile on all operating systems
  • read_topology_linux() doesn't contain any C++ code that would prevent the code from compiling on non-Linux operating systems
  • A downside is the increase in code segment size in libcpugraph.so on non-Linux OSes, if the compiler & linker are unable to remove read_topology_linux() during dead code elimination

Closes: #28 (closed)