Allow ALTER FUNCTION to change a function's strictness, volatility, and

whether or not it is a security definer. Changing a function's strictness
is required by SQL2003, and the other capabilities make sense. Also, allow
an optional RESTRICT noise word to be specified, for SQL conformance.

Some trivial regression tests added and the documentation has been
updated.

src/test/regress/expected/alter_table.out

@@ -1235,3 +1235,38 @@ select * from another;
 (3 rows)
 
 drop table another;
+--
+-- alter function
+--
+create function test_strict(text) returns text as
+    'select coalesce($1, ''got passed a null'');'
+    language sql returns null on null input;
+select test_strict(NULL);
+ test_strict 
+-------------
+ 
+(1 row)
+
+alter function test_strict(text) called on null input;
+select test_strict(NULL);
+    test_strict    
+-------------------
+ got passed a null
+(1 row)
+
+create function non_strict(text) returns text as
+    'select coalesce($1, ''got passed a null'');'
+    language sql called on null input;
+select non_strict(NULL);
+    non_strict     
+-------------------
+ got passed a null
+(1 row)
+
+alter function non_strict(text) returns null on null input;
+select non_strict(NULL);
+ non_strict 
+------------
+ 
+(1 row)
+

src/test/regress/sql/alter_table.sql

@@ -975,3 +975,20 @@ alter table another
 select * from another;
 
 drop table another;
+
+--
+-- alter function
+--
+create function test_strict(text) returns text as
+    'select coalesce($1, ''got passed a null'');'
+    language sql returns null on null input;
+select test_strict(NULL);
+alter function test_strict(text) called on null input;
+select test_strict(NULL);
+
+create function non_strict(text) returns text as
+    'select coalesce($1, ''got passed a null'');'
+    language sql called on null input;
+select non_strict(NULL);
+alter function non_strict(text) returns null on null input;
+select non_strict(NULL);