test-charset-fidelity: set iocharset in vfat/msdos FSes

The default charset for these filesystems depends on how the kernel was
configured with, so explicitly set the default one (as specified in
Linux sources), eventually setting later a new one for the tests.
resize: print FSes which cannot be expanded

Should ease a bit the discovery of a filesystem which cannot be
expanded.
fish: fix small memory leak in completion

Do not leak the small memory buffer with the path.