networkd-util: Avoid call to endswith()

networkd-util: Avoid call to endswith()

Instead of recalculating the length of the string again after
reading it, let's make sd_netlink_message_read_string() return the
length of the string that we then use to check if the last character
is a dot or not.

This allows us to get rid of the string-util.h include in #37344.