- add OpenBSD to list of recognized BSDs.

USB (and probably MTP) support needs more work, the current code only
supports OSX, Windows, and Linux SYSFS.

Index: src/calibre/constants.py
--- src/calibre/constants.py.orig
+++ src/calibre/constants.py
@@ -28,7 +28,8 @@ isnewosx  = ismacos and getattr(sys, 'new_app_bundle',
 isfreebsd = 'freebsd' in _plat
 isnetbsd = 'netbsd' in _plat
 isdragonflybsd = 'dragonfly' in _plat
-isbsd = isfreebsd or isnetbsd or isdragonflybsd
+isopenbsd = 'openbsd' in _plat
+isbsd = isfreebsd or isnetbsd or isdragonflybsd or isopenbsd
 ishaiku = 'haiku1' in _plat
 islinux   = not (iswindows or ismacos or isbsd or ishaiku)
 isfrozen  = hasattr(sys, 'frozen')
@@ -280,7 +281,7 @@ class ExtensionsImporter:
             extra = ('winutil', 'wpd', 'winfonts', 'wintoast')
         elif ismacos:
             extra = ('usbobserver', 'cocoa', 'libusb', 'libmtp')
-        elif isfreebsd or ishaiku or islinux:
+        elif isfreebsd or ishaiku or islinux or isopenbsd:
             extra = ('libusb', 'libmtp')
         else:
             extra = ()
