coredump-context: several cleanups

coredump-context: several cleanups

- Renames Context to CoredumpContext.
- Renames enum meta_argv_t to MetadataField, and use the correct type at
  several pleaces, especially in loop.
- Parses argv (when invoked as kernel helper or with '--backtrace') or
  iov (when invoked through socket) and store values in the context.
  Previously we use the passed string as is, but let's make them in a
  type safe way.
- Stores received or built iovw in the context. Also store input fd and
  mountfs fd in the context.
- Introduces coredump_context_is_pid1() and _is_journald() helper
  functions.
- Adds COREDUMP_PIDFDID= field when the kernel passed pidfd of the
  crashed process.

No effective funtional change. Just refactoring.