Backwards compatible fixes to bpf() syscall emulation

[lmb]

bpf(): do not return errors via errno

The Linux ABI returns all syscall errors via the function return value, not
via errno.

Fixes https://github.com/microsoft/ebpf-for-windows/issues/3749

Allow detecting if bpf() command is not implemented

Use SetLastError to indicate to callers that a bpf() command is not 
implemented. This avoids polluting the bpf() return value with platform
specific error returns while still allowing detection of this important
case.

Add forwards and backwards compatibility to bpf() emulation

On Linux, bpf() accepts a bpf_attr which is larger than what the syscall
expects, as long as the unknown fields are all 0. It also accepts a bpf_attr
which is smaller than what it expects, by assuming that all missing fields
are zero.

This allows forwards and backwards compatilibity between old and new 
versions of both the Linux kernel and user space tooling.

Implement a similar scheme for the bpf() emulation.

Return EPERM from bpf() if user is not privileged

On Linux, bpf() returns EPERM if the user doesn't have CAP_BPF. Return the
same error when the user isn't able to open the device handle.

[dthaler]

Looks fine other than two style nits