Revive line type

Change the input/output format to {A,B,C}, to match the internal
representation.

Complete the implementations of line_in, line_out, line_recv, line_send.
Remove comments and error messages about the line type not being
implemented.  Add regression tests for existing line operators and
functions.

Reviewed-by: rui hua <365507506hua@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@2ndquadrant.com>
Reviewed-by: Jeevan Chalke <jeevan.chalke@enterprisedb.com>

src/test/regress/sql/geometry.sql

@@ -58,10 +58,6 @@ SELECT '' AS count, p.f1, l.s, l.s # p.f1 AS intersection
 SELECT '' AS thirty, p.f1, l.s, p.f1 ## l.s AS closest
    FROM LSEG_TBL l, POINT_TBL p;
 
---
--- Lines
---
-
 --
 -- Boxes
 --