AutoLISP: entmake defines blocks with BLOCK … ENDBLK (fixes #1662) (#1765)

* AutoLISP: entmake defines blocks with BLOCK ... ENDBLK (fixes #1662)

A BLOCK list wasn't an entity, so entmake returned nil and the entities
after it went into model space. Now a BLOCK list begins a definition
(name, flags, base point, description), the entities made meanwhile go
into it, and ENDBLK adds the block (redefining one of the same name, or
naming an anonymous *U block) and returns its name. An open definition,
an xref, an invalid name or a block that inserts itself is refused with
nil, using BLOCK's own name and self-reference checks.

* AutoLISP entmake blocks: review fixes (anonymous names keep a star, no redefinition during REFEDIT, a failed routine drops its open definition)

An anonymous block (flag 1) named without a * got a name like FO1; it now
gets *U. ENDBLK refuses to redefine a block REFEDIT is editing, as BLOCK
does. A routine that fails drops the definition it began, so the next
routine's entities go into the drawing; one that ends normally still
leaves it open.

* AutoLISP entmake blocks: refuse an anonymous block that inserts itself

The self-reference check ran only for named blocks, so an anonymous *U
block holding an INSERT of the name it was about to get (*U1 in an empty
drawing) was stored. The check now runs after the anonymous name is
given, for both kinds.

* AutoLISP entmake blocks: create a definition's layers only when the block is added

Each pending entity created its layer at once, so a definition ENDBLK
refused (a self-reference, REFEDIT) or a failed routine dropped left an
orphan layer. end_block now creates the layers after its checks pass,
just before it adds the block.

docs/autolisp.md

@@ -82,6 +82,14 @@ DXF file holds for the object, plus `-1` (its name) and `410` (its layout). Angl
 are in radians, as in AutoCAD, where the file holds degrees; `entmake`, `entmod` and `ssget`
 filters take radians too.
 
+`entmake` defines blocks: a BLOCK list (2 name, 70 flags, 10 base point, 4 description) begins a
+definition, the entities made after it go into the block instead of the drawing, and an ENDBLK
+list adds the block (redefining one of the same name) and returns its name. Flag 1 with a `*U`
+name makes an anonymous block, named when it ends. A BLOCK while a definition is open, an
+xref (flag 4), an invalid name, a block that inserts itself and a block being edited in place
+(REFEDIT) return nil. A definition stays open across routines until ENDBLK, but a routine that
+fails drops the one it began.
+
 ## Testing against the corpus
 
 The AutoLISP conformance corpus (#648) lives in its own repository. Point `CADCRAFT_LISP_CORPUS`