exec-invoke: respect $HOME set via PAM

exec-invoke: respect $HOME set via PAM

This follows the same recent change in util-linux:

https://github.com/util-linux/util-linux/pull/3354

i.e. we generally want that PAM modules can override $HOME and it is
honoured for the CWD after login.

(This renames the 'home' variable we maintained sofar to 'pwent_home',
to clarify that it's the home directory listed in the struct passwd
entry, and thus not necessarily the one actually used)