unit: printf specifiers %u and %h: $USER and $HOME.

These printf specifiers allow us to refer to $HOME and $USER
in unit files. These are particularly helpful in instanced
units that have "User=" set, and in systemd --user domains.

The specifiers will return the pw_name and pw_dir fields
if the unit file has a User= field.

If the unit file does not have a User= field, the value
substituted is either $USER or $HOME from the environment,
or, if unset, the values from pw_name or pw_dir.

This patch is somewhat after Ran Benita's original patch,
which didn't get merged. I've split up the 2 specifiers
and extended them to do what is logically expected from
these specifiers.

Note that expansion is done at `start` time, not after
the units are parsed. Using `systemctl show` will just
show the specifiers.