Split up the giant switch statement in homectl's parse_argv (#40059)

This is in preparation for later changes. The 1500+ line switch statement was very
hard to work with and had quite a bit of duplicated code. This is a rework that
splits outs parts into functions. I split this up into a bunch of commits so that it
can be reasonably reviewed. When viewed as one giant commit, diff just shows
a giant block of removed code and a giant block of added code.

In some cases I wasn't sure why one or the other of the identity arrays is used…
I preserved existing behaviour in all cases.

diff --cc src/home/homectl.c
Simple merge