Fix `auto_unmount` to work without `allow_other`

In
https://github.com/libfuse/libfuse/blob/77d662459a0fcdf358d515477d33795837e859d5/util/fusermount.c#L1219
`open` is executed as root which does not have access to the mount
point if `allow_other` was not used and the real user id is not 0. Since
`allow_other` usually cannot be specified by unprivileged users,
`auto_unmount` has no effect for unprivileged users.

In this commit, we work around this limitation:

We first try to open the mountpoint as root, and if we get `EACCES`, we
retry as the user who started fusermount, and see if we get `ENOTCONN`.

In my testing, I found that `setfsuid` and `setfsgid` don't work to get
around the lack of `allow_other`.  (Sorry, I don't know enough about the
Linux kernel to tell whether that's significant.)  As a workaround, I
decided to use `setresuid` and `setresgid` in a forked child process,
and communicate via its exit status.

Please give feedback on correctness, style and suggest tests.

Fixes https://github.com/libfuse/libfuse/issues/586

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