mounts: Remove `optional` from mount tree proto

Proto3 semantics are "optional" by default and now only control generation of `has_XXX()` presence checks. For the mount tree, we only need those for `node`.

PiperOrigin-RevId: 683103568
Change-Id: I3c83385b52431c135df518d21cb20267beb09bf0

Add `DefaultAction(TraceAllSyscalls)` variant to `PolicyBuilder`

This helps write the kind of 'log, but allow' policy described in
[`notify.h`](https://github.com/google/sandboxed-api/blob/b9c84a1f75d282ff38ebed068bebe9960af9eb71/sandboxed_api/sandbox2/notify.h#L57) for all system calls not mentioned explicitly. One use case is writing a "permissive mode" runtime to give more information during development.

PiperOrigin-RevId: 603766051
Change-Id: I3c72f433a1e21c330b5dd9f1ede2faa570b75b09

Ensure compatibility with CMake 3.10

This fixes some CMake-3.12+-isms, like `list(JOIN ...)` and setting link
properties on OBJECT libraries.

PiperOrigin-RevId: 265033725
Change-Id: I0ee1ec0e1b1097ea8226ad6fdeff794a97c2881b

Sandboxed API OSS release.

PiperOrigin-RevId: 238996664
Change-Id: I9646527e2be68ee0b6b371572b7aafe967102e57

Signed-off-by: Christian Blichmann <cblichmann@google.com>