fix GitHub issue 1303 related to engravings

GitHub issue reported by ars3niy:
https://github.com/NetHack/NetHack/issues/1303

@ars3niy commented on Oct 27:
Stepping on a square with a dust or "graffiti" engraving while blind
produces no message because presumably you can't read them by swiping
the floor with your hands, however the engraving glyph still shows up on
the map afterwards. While this helps zen players, it looks like a bug.

@ville-v commented 3 days ago:
Searching while blind also reveals the engravings. Here is a save file
demonstrating the issue.
[...]

This adds an erevealed bit to engravings, to accompany the the eread
bit that is already there.

eread:      refers to the text of the engraving
erevealed:  refers to the engraving map symbol

Hopefully, this resolves issue 1303 without creating additional bugs.

This invalidates existing save files and bones.

Fixes #1303

include/engrave.h

@@ -33,8 +33,9 @@ struct engr {
                                 * even when hero isn't (so behaves similarly
                                 * to how Elbereth did in 3.4.3) */
     Bitfield(nowipeout, 1);    /* this engraving will not degrade */
-    Bitfield(eread, 1);        /* the engraving text has been read or felt */
-    /* 5 free bits */
+    Bitfield(eread, 1);        /* refers to the engaving text: read or felt */
+    Bitfield(erevealed, 1);    /* refers to engraving map symbol: revealed */
+    /* 4 free bits */
 };
 
 #define newengr(lth) \

include/extern.h

@@ -969,6 +969,7 @@ extern void make_grave(coordxy, coordxy, const char *);
 extern void disturb_grave(coordxy, coordxy);
 extern void see_engraving(struct engr *) NONNULLARG1;
 extern void feel_engraving(struct engr *) NONNULLARG1;
+extern boolean engr_can_be_felt(struct engr *) NONNULLARG1;
 
 /* ### exper.c ### */