basic/unit-name: do not use strdupa() on a path
    
    The path may have unbounded length, for example through a fuse mount.
    
    CVE-2021-33910: attacked controlled alloca() leads to crash in systemd and
    ultimately a kernel panic. Systemd parses the content of /proc/self/mountinfo
    and each mountpoint is passed to mount_setup_unit(), which calls
    unit_name_path_escape() underneath. A local attacker who is able to mount a
    filesystem with a very long path can crash systemd and the whole system.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1970887
    
    The resulting string length is bounded by UNIT_NAME_MAX, which is 256. But we
    can't easily check the length after simplification before doing the
    simplification, which in turns uses a copy of the string we can write to.
    So we can't reject paths that are too long before doing the duplication.
    Hence the most obvious solution is to switch back to strdup(), as before
    7410616cd9dbbec97cf98d75324da5cda2b2f7a2.
    
    (cherry picked from commit 441e0115646d54f080e5c3bb0ba477c892861ab9)
    (cherry picked from commit 764b74113e36ac5219a4b82a05f311b5a92136ce)