[fsstorage] use unique filenames for PUOID databases

All instances of FsStoragePlugin were using file ~/.local/mtp/mtppuoids,
overwriting each other's data. The new naming scheme is

 mtppuoids-{name}-{fsuuid}

Where name is a storage name attribute from its configuration file in
/etc/fsstorage.d and fsuuid is a UUID of storage's filesystem.

UUID alone is not enough for unique distinction since different parts of
a single filesystem may be exported as different MTP storages. In that
case, user-configured storage names ensure unique database filenames.
[fsstorage] remove legacy PUOID database

The file used in previous versions of buteo-mtp now only wastes device
memory.
[fsstorage] ensure storage path exists when instantiating the plugin
[tests] don't link storage-test with libblkid and libmount

Avoid storage-test crash on device:

[W] QCoreApplicationPrivate::QCoreApplicationPrivate:416 - WARNING: \
QApplication was not created in the main() thread.
Segmentation fault (core dumped)

It's probably caused by TLS variable currentThreadData in Qt's
qthread_unix.cpp not getting initialized to zero, which seems like a
compiler or run-time linker issue. Once sorted out, this commit may be
reverted.