convert string_vector_(iter(free) + reset()) to string_vector_empty()

convert string_vector_(iter(free) + reset()) to string_vector_empty()

(Reimplement libnbd commit c7ff70101e8c, "convert
string_vector_(iter(free) + reset()) to string_vector_empty()",
2023-02-28.)

Convert the 5 string_vector_(iter(free) + reset()) call sites mentioned
previously to string_vector_empty().

Note that the converted code performs more cleanup steps in some cases
than strictly necessary, but the extra work is harmless, and arguably
beneficial for clarity / consistency.

Also note that the CLEANUP_FREE_*() macros remain unique to nbdkit; libnbd
does not use __attribute__((cleanup)), per libnbd commit f306e231d294
("common/utils: Add extensible string, based on vector", 2022-03-12).

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20230303075145.177338-10-lersek@redhat.com>
Acked-by: Richard W.M. Jones <rjones@redhat.com>
Acked-by: Eric Blake <eblake@redhat.com>