fusermount is (usually) from libfuse v2, and fusermount3 is from
libfuse v3, which is the actively-developed version.

Importantly, libfuse v3 added ExFAT to the list of filesystems
that users are allowed to mount on, and libfuse v2 did not.

The only ABI difference between fusermount and fusermount3 that
I am aware of is that fusermount3 dropped the allow_empty option.

Filesystems had to deal with that already, as fusermount may
also be from libfuse v3, depending on what the distro does.

This commit also adds two lines of debug output about which binary
we actually call.

Passes all.bash & the gocryptfs test suite.

Fixes https://github.com/hanwen/go-fuse/issues/394
Fixes https://github.com/rfjakob/gocryptfs/issues/626

Change-Id: Id4574fb9c8d2c812a524181a76616159256d551c