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/path.c

@@ -2445,7 +2445,7 @@ void path_guess_exepath(const char *argv0, char *buf, size_t bufsize)
       if (dir_len + 1 > sizeof(NameBuff)) {
         continue;
       }
-      xstrlcpy(NameBuff, dir, dir_len + 1);
+      xmemcpyz(NameBuff, dir, dir_len);
       xstrlcat(NameBuff, PATHSEPSTR, sizeof(NameBuff));
       xstrlcat(NameBuff, argv0, sizeof(NameBuff));
       if (os_can_exe(NameBuff, NULL, false)) {