Remove support for postfix (right-unary) operators.

This feature has been a thorn in our sides for a long time, causing
many grammatical ambiguity problems.  It doesn't seem worth the
pain to continue to support it, so remove it.

There are some follow-on improvements we can make in the grammar,
but this commit only removes the bare minimum number of productions,
plus assorted backend support code.

Note that pg_dump and psql continue to have full support, since
they may be used against older servers.  However, pg_dump warns
about postfix operators.  There is also a check in pg_upgrade.

Documentation-wise, I (tgl) largely removed the "left unary"
terminology in favor of saying "prefix operator", which is
a more standard and IMO less confusing term.

I included a catversion bump, although no initial catalog data
changes here, to mark the boundary at which oprkind = 'r'
stopped being valid in pg_operator.

Mark Dilger, based on work by myself and Robert Haas;
review by John Naylor

Discussion: https://postgr.es/m/38ca86db-42ab-9b48-2902-337a0d6b8311@2ndquadrant.com

src/tutorial/complex.source

@@ -111,7 +111,7 @@ CREATE FUNCTION complex_add(complex, complex)
    LANGUAGE C IMMUTABLE STRICT;
 
 -- we can now define the operator. We show a binary operator here but you
--- can also define unary operators by omitting either of leftarg or rightarg.
+-- can also define a prefix operator by omitting the leftarg.
 CREATE OPERATOR + (
    leftarg = complex,
    rightarg = complex,

src/tutorial/syscat.source

@@ -96,36 +96,22 @@ SELECT n.nspname, r.rolname, format_type(t.oid, null) as typname
 
 
 --
--- lists all left unary operators
+-- lists all prefix operators
 --
-SELECT n.nspname, o.oprname AS left_unary,
+SELECT n.nspname, o.oprname AS prefix_op,
        format_type(right_type.oid, null) AS operand,
        format_type(result.oid, null) AS return_type
   FROM pg_namespace n, pg_operator o,
        pg_type right_type, pg_type result
   WHERE o.oprnamespace = n.oid
-    and o.oprkind = 'l'           -- left unary
+    and o.oprkind = 'l'           -- prefix ("left unary")
     and o.oprright = right_type.oid
     and o.oprresult = result.oid
   ORDER BY nspname, operand;
 
 
 --
--- lists all right unary operators
---
-SELECT n.nspname, o.oprname AS right_unary,
-       format_type(left_type.oid, null) AS operand,
-       format_type(result.oid, null) AS return_type
-  FROM pg_namespace n, pg_operator o,
-       pg_type left_type, pg_type result
-  WHERE o.oprnamespace = n.oid
-    and o.oprkind = 'r'          -- right unary
-    and o.oprleft = left_type.oid
-    and o.oprresult = result.oid
-  ORDER BY nspname, operand;
-
---
--- lists all binary operators
+-- lists all infix operators
 --
 SELECT n.nspname, o.oprname AS binary_op,
        format_type(left_type.oid, null) AS left_opr,
@@ -134,7 +120,7 @@ SELECT n.nspname, o.oprname AS binary_op,
   FROM pg_namespace n, pg_operator o, pg_type left_type,
        pg_type right_type, pg_type result
   WHERE o.oprnamespace = n.oid
-    and o.oprkind = 'b'         -- binary
+    and o.oprkind = 'b'         -- infix ("binary")
     and o.oprleft = left_type.oid
     and o.oprright = right_type.oid
     and o.oprresult = result.oid