In lib/global-functions.sh added
new function get_var_from_file()
see #3171

Dropped the  final '|| return 1' from the
get_var_from_file() implementation, see
#3203 (comment)

Explain why we source the file in a separated shell
and why stdout of the sourced file must be discarded
and add specific URLs to pull request comments
that show our resoning behind

Typo fix "the the" -> "the"

Typo fix in comment: Removed false full stop '.'
because the sentence does not end there.

Typo fix in comment "interits" -> "inherits"

Renamed get_var_from_file into get_shell_file_config_variable
to tell what that function actually is about, see
#3203 (comment)

Renamed get_shell_file_config_variable
into source_variable_from_file
to make it explicit that the file is sourced.

Added new helper function is_trustworthy_for_root
to check if only 'root' could have written a file
which makes such files trustworthy to be used by ReaR
and call that helper function in source_variable_from_file(), see
#3203 (comment)

Fixed is_trustworthy_for_root:
Only check that the owner name is 'root' because
the group does not matter when it has no write permissions.
Treat files with an ACL as untrustworthy to be on the safe side
because (at last currently) ACLs are not checked.
See #3203 (comment)