loader.py: Separate functions for each operating system

[cclauss]

Separate functions for each operating system.

+ def _lib_candidates_linux():
+     """Yield possible libmagic library names on Linux."""
+
+ def _lib_candidates_macos():
+     """Yield possible libmagic library names on macOS."""
+
+ def _lib_candidates_windows():
+     """Yield possible libmagic library names on Windows."""

Related to:

• initial smartos support #132
• bugfix: add path to load libmagic bin on Windows #249
• Build platform-specific wheels containing libmagic #294

[ahupp]

Seems reasonable, thanks!