Pass rlimits to urunit

[cmainas]

As part of setting up the execution environment inside the Linux VM, we also need to set up the rlimits for the process. In particular, we need to pass the information of rlimits from the container configuration to urunit and the urunit should set them up.

This should be done in a similar way as other information passed in #253

[codesmith25103]

Hey @cmainas can you provide some more detail to solve it?

[cmainas]

Hello @codesmith25103 , according to the OCI spec, a container configuration might indicate that we need to set some specific resource limits in the container process (See: https://github.com/opencontainers/runtime-spec/blob/main/config.md#posix-process). However, in urunc, where the process executes inside a sandbox, we need to pass this information inside the sandbox and set it for the process. This feature is only possible for Linux-based sandboxes (normal containers running inside a Linux VM), since most unikernels do not provide such support.

Therefore, in a similar way as #253 we need to pass the rlimits information from the container configuration inside the Linux VM and let urunit set it. In particular, we need to discover the respective field in the container configuration (https://pkg.go.dev/github.com/opencontainers/runtime-spec/specs-go#POSIXRlimit) and write this information in a file that will be later passed to the VM.

[namansh70747]

Working implementation submitted in #558.

Includes serialization of OCI rlimits from the container spec into
URUNIT_CONFIG (as RLIMIT:<type>:<soft>:<hard> entries), unit tests,
and spell-check dictionary update.

Companion urunit PR (reading and applying the limits via setrlimit(2)
before privilege drop): nubificus/urunit#14