^G of unseen monster

I used ^G to create a monster and specified "invisible owlbear".  I
then got "An owlbear appears next to you."  Except it didn't; it was
invisible and I lacked see-invisible.  I imagine that newsym() was
called for the new-yet-invisible monster, but that remained buffered
and was gone overridden by the time pending map update got flushed
at some point after the monster was made invisible.

Add a new makemon() flag to turn a newly created monster invisible
during its creation, before "monster appears" message is delivered.
Since that message will now be suppressed in this situation, use the
cursor-flash hack that indicates where the new, unseen monster got
placed.  Creating "1000 invisible <mon>" is something you probably
won't do twice.

include/hack.h

@@ -1043,6 +1043,7 @@ typedef uint32_t mmflags_nht;     /* makemon MM_ flags */
 #define GP_CHECKSCARY 0x200000L /* check monster for onscary() */
 #define MM_NOEXCLAM   0x400000L /* more sedate "<mon> appears." mesg for ^G */
 #define MM_IGNORELAVA 0x800000L /* ignore lava when positioning */
+#define MM_MINVIS   0x01000000L /* for ^G/create_particular */
 
 /* flags for make_corpse() and mkcorpstat(); 0..7 are recorded in obj->spe */
 #define CORPSTAT_NONE     0x00