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/vim.h

@@ -1024,7 +1024,7 @@ typedef enum {
 #define EMSG3(s, p, q)              emsg3((char_u *)(s), (char_u *)(p), \
     (char_u *)(q))
 #define EMSGN(s, n)                 emsgn((char_u *)(s), (int64_t)(n))
-#define EMSGU(s, n)                 emsgu((char_u *)(s), (long_u)(n))
+#define EMSGU(s, n)                 emsgu((char_u *)(s), (uint64_t)(n))
 #define OUT_STR(s)                  out_str((char_u *)(s))
 #define OUT_STR_NF(s)               out_str_nf((char_u *)(s))
 #define MSG_PUTS(s)                 msg_puts((char_u *)(s))