Use portable format specifiers: Adapt/fix EMSGU/emsgu to use (uint64_t).

Problem

EMSGU macro was defined as the unsigned counterpart of EMSGN. This is,
invoking emsgu instead of emsgn and having a long_u argument instead of
a long.
But, surprisingly, emsgu was not defined anywhere, which didn't result
in an error because in fact EMSGU was not used (the only point in code
printing a %lu erroneously using EMSGN instead).

Solution

- Define emsgu.
- Adapt EMSGU macro to use uint64_t instead of long_u.

src/misc2.c

@@ -899,6 +899,17 @@ int emsgn(char_u *s, int64_t n)
   return emsg(IObuff);
 }
 
+/*
+ * Print an error message with one "%" PRIu64 and one (uint64_t) argument.
+ */
+int emsgu(char_u *s, uint64_t n)
+{
+  if (emsg_not_now())
+    return TRUE;                /* no error messages at the moment */
+  vim_snprintf((char *)IObuff, IOSIZE, (char *)s, n);
+  return emsg(IObuff);
+}
+
 /*
  * Read 2 bytes from "fd" and turn them into an int, MSB first.
  */