Bug 1077057 - Expose Linux sandboxing information to JS via nsSystemInfo.

This adds "hasSeccompBPF" for seccomp-bpf support; other "has" keys
will be added in the future (e.g., user namespaces).

This also adds "canSandboxContent" and "canSandboxMedia", which are
absent if the corresponding type of sandboxing isn't enabled at build
type (or is disabled with environment variables), and otherwise present
as a boolean indicating whether that type of sandboxing is supported.
Currently this is always the same as hasSeccompBPF, but that could change
in the future.

Some changes have been made to the "mozilla/Sandbox.h" interface to
support this; the idea is that the MOZ_DISABLE_*_SANDBOX environment
variables should be equivalent to disabling MOZ_*_SANDBOX at build type.
Add Linux sandboxing info to telemetry.
Add Linux sandboxing info to FHR sysinfo.
Bug 1077057 - Display Linux sandboxing information as about:sandbox.