If libbtctl can't initialise the Bluetooth device, then nautilus-sendto will crash on startup (usually, while loading the interface).

The problem is that if libbtctl fails, it will create a new GQuark using g_quark_from_static_string. And the GModule documentation says:
"
If your module introduces static data to common subsystems in the running program, e.g. through calling g_quark_from_static_string ("my-module-stuff"), it must ensure that it is never unloaded, by calling g_module_make_resident().
"

So using g_module_close on the bluetooth plugin will remove a quark from a static string, and crash nautilus-sendto whenever another quark is registered.

The attached patch adds the "never_unload" struct member so that modules that say so can't be unloaded.

It also fixes:
- don't use home-made str_has_suffix, we have g_str_has_suffix
- use g_dir_open instead of opendir
- don't leak the plugins directory on startup
- mark as static functions that are not used anywhere else
- namespace the plugins so as to avoid clashes with system libraries, confusing the hell out of libtool (libbluetooth.so is the generated library, as well as the system bluetooth library)
- use GError properly in the bluetooth module
- Fix a couple of warnings in the evo plugin