…nager

This patch changes the way user managers set the default umask for the units it
manages.

Indeed one can expect that if user manager's umask is redefined through PAM
(via /etc/login.defs or pam_umask), all its children including the units it
spawns have their umask set to the new value.

Hence make user units inherit their umask value from their parent instead of
the hard coded value 0022 but allow them to override this value via their unit
file.

Note that reexecuting managers with 'systemctl daemon-reexec' after changing
UMask= has no effect. To take effect managers need to be restarted with
'systemct restart' instead. This behavior was already present before this
patch.

Fixes #6077.

(cherry picked from commit 5e37d19)

Related: RHEL-28048

Setting umask for user sessions via UMASK setting in /etc/login.defs is
a well-known feature. Let's make sure that user manager also runs with
this umask value.

Follow-up for 5e37d19.

(cherry picked from commit 159f1b78576ce91c3932f4867f07361a530875d3)

Resolves: RHEL-28048