cutils: Allow NULL endptr in parse_uint()

cutils: Allow NULL endptr in parse_uint()

All the qemu_strto*() functions permit a NULL endptr, just like their
libc counterparts, leaving parse_uint() as the oddball that caused
SEGFAULT on NULL and required the user to call parse_uint_full()
instead.  Relax things for consistency, even though the testsuite is
the only impacted caller.  Add one more unit test to ensure even
parse_uint_full(NULL, 0, &value) works.  This also fixes our code to
uniformly favor EINVAL over ERANGE when both apply.

Also fixes a doc mismatch @v vs. a parameter named value.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
Message-Id: <20230522190441.64278-9-eblake@redhat.com>