Use unikernel arguments section

[reynir]

See mirage/mirage#1531

[hannesm]

I suggest two things:

• let's bind "let docs = Mirage_runtime.s_arg" and use ~docs in all the keys (this way, we don't introduce another indirection/function for no obvious good reason)
• Also, in config.ml we should use the Mirage_runtime.s_log for the respective keys...

[hannesm]

I integrated part into 5d5b7c3, and since mirage 4.5.1 now does a reasonable ordering of the sections, no need to put each argument into a specific section.