refactor: add xmemcpyz() and use it in place of some xstrlcpy() (#28422)

Problem:  Using xstrlcpy() when the exact length of the string to be
          copied is known is not ideal because it requires adding 1 to
          the length and an unnecessary strlen().
Solution: Add xmemcpyz() and use it in place of such xstrlcpy() calls.

src/nvim/eval/userfunc.c

@@ -345,7 +345,7 @@ int get_lambda_tv(char **arg, typval_T *rettv, evalarg_T *evalarg)
     char *p = xmalloc(len);
     ((char **)(newlines.ga_data))[newlines.ga_len++] = p;
     STRCPY(p, "return ");
-    xstrlcpy(p + 7, start, (size_t)(end - start) + 1);
+    xmemcpyz(p + 7, start, (size_t)(end - start));
     if (strstr(p + 7, "a:") == NULL) {
       // No a: variables are used for sure.
       flags |= FC_NOARGS;