port document portal to libfuse3

[ahkok]

By default, fuse installs fusermount3 and not fusermount. Any chance we can get a --with-fusermount=fusermount3 configure option or similar so we can account for this difference?

Reference:
https://github.com/libfuse/libfuse/blob/master/util/meson.build

[jhenstridge]

The fusermount utility is provided by libfuse 2.x, which is intended to be parallel installable with libfuse3.

Eventually the document portal should be ported to fuse3, but that is a bit more involved than just s/fusermount/fusermount3/.

[nullr0ute]

Would be nice to get this revisited, the last release of fuse2 was 2.9.9 in Jan 2019 so fuse2 is no longer maintained or getting security releases.

[ovitters]

An overview of the API changes was published with the fuse 3.0 release notes: https://github.com/libfuse/libfuse/releases/tag/fuse-3.0.0