Major amnesia revamp

Instead of forgetting maps and objects, make amnesia forget skills.
Forgetting maps and objects could be circumvented with taking notes,
or by using an external tool to remember the forgotten levels.

Forgetting skills allows the player to optionally go down another
skill path, if they trained the wrong weapon in the early game.

Amnesia still forgets spells.

As a replacement for the deja vu messages when entering a forgotten
level, those messages will now indicate a ghost with your own name
existing on the level, given only when the level is entered for
the first time.

These changes based on fiqhack, with some adjustments.

include/dungeon.h

@@ -159,7 +159,7 @@ typedef struct branch {
 struct linfo {
     unsigned char flags;
 #define VISITED 0x01      /* hero has visited this level */
-#define FORGOTTEN 0x02    /* hero will forget this level when reached */
+/* 0x02 was FORGOTTEN, when amnesia made you forget maps */
 #define LFILE_EXISTS 0x04 /* a level file exists for this level */
         /* Note:  VISITED and LFILE_EXISTS are currently almost always
          * set at the same time.  However they _mean_ different things.