Use ensure_last_os_error() with libfuse2

[str4d]

Under some conditions, fuse_mount_compat25 may return -1 to indicate
an error but does not set errno. io::Error::last_os_error() will then
be equivalent to Err(Success) which is somewhat surprising.

In particular, if the mount options auto_unmount,allow_{root,other}
are set but the user does not have user_allow_other set in their
fuse.conf, the fusermount binary will print an error message to
stderr but not set errno.

fusermount3 has the same behaviour, but this issue was mitigated for
the libfuse3 binding in #178 by mapping Err(Success) to
the slightly more useful Error(Other, "Unspecified Error"). This
commit applies the same fix to the libfuse2 binding.

[str4d]

The CI / ci (pre) step is failing because of an unrelated change to the license specifier in unicode-ident 1.0.3 (dtolnay/unicode-ident#9) to correctly note that anything generated from the Unicode Character Database needs to be used under Unicode-DFS-2016.

[cberner]

I merged a fix for that. Could you rebase on master and force push your branch to restart the CI?

[str4d]

Rebased on master to fix CI. It won't automatically restart though because I still count as a new contributor, so someone needs to check I didn't insert a cryptominer during the rebase 🙃

[cberner]

Merged. Thanks!