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/ex_session.c

@@ -1138,7 +1138,7 @@ static char *get_view_file(char c)
   }
   *s++ = '=';
   *s++ = c;
-  xstrlcpy(s, ".vim", 5);
+  xmemcpyz(s, S_LEN(".vim"));
 
   xfree(sname);
   return retval;