ptp-helper: Rewrite in Rust for portability and security

This works on Linux, Android, Windows, macOS, FreeBSD, NetBSD, OpenBSD,
DragonFlyBSD, Solaris and Illumos.
Newly supported compared to the C version is Windows.

Compared to the C version various error paths are handled more correctly
and a couple of memory leaks are fixed. Otherwise it should work identically.

The minimum required Rust version for compiling this is 1.48, i.e. the
version currently in Debian stable. On Windows, Rust 1.54 is needed at
least.

Fixes gstreamer/gstreamer#1259