Find fusermount on the $PATH

[probonopd]

We cannot rely on the compile-time FUSERMOUNT_DIR at build time but have to look at the $PATH on the target system at runtime, because distributions agree on nothing.

Use case:

• How to launch without FUSERMOUNT_PROG variable? probonopd/go-appimage#281

Essentially we need to reinstate findBinaryOnPath from f0e35b0.

[TheAssassin]

Can't we send the required syscall(s) directly without requiring an external utility to be present on the system?

[probonopd]

That'd be awesome. But the way I understand it that'd mean that we'd have to be setuid root, which is obviously not an option. (They always say FUSE is "in userland", but it still requires code that runs as root - that is fusermount.)