refactor: Remove strncpy/STRNCPY. (#6008)

Closes #731
References #851

Note: This does not remove some intentional legacy usages of strncpy.
      - memcpy isn't equivalent because it doesn't check the string
        length of `src`, and doesn't zero-out the remainder of `dst`.
      - xstrlcpy isn't equivalent because it doesn't zero-out the
        remainder of `dst`. Some Vim logic depends on that (e.g.
        ex_append which calls vim_strnsave).

Helped-by: Douglas Schneider <ds3@ualberta.ca>
Helped-by: oni-link <knil.ino@gmail.com>
Helped-by: James McCoy <jamessan@jamessan.com>

src/nvim/ex_docmd.c

@@ -3854,7 +3854,7 @@ static char_u *replace_makeprg(exarg_T *eap, char_u *p, char_u **cmdlinep)
       ptr = new_cmdline;
       while ((pos = (char_u *)strstr((char *)program, "$*")) != NULL) {
         i = (int)(pos - program);
-        STRNCPY(ptr, program, i);
+        memcpy(ptr, program, i);
         STRCPY(ptr += i, p);
         ptr += len;
         program = pos + 2;