Fix lockmode initialization for custom relation options

The code was enforcing AccessExclusiveLock for all custom relation
options, which is incorrect as the APIs allow a custom lock level to be
set.

While on it, fix a couple of inconsistencies in the tests and the README
of dummy_index_am.

Oversights in commit 773df883.

Discussion: https://postgr.es/m/20190925234152.GA2115@paquier.xyz

src/backend/access/common/reloptions.c

@@ -700,13 +700,6 @@ allocate_reloption(bits32 kinds, int type, const char *name, const char *desc,
 	newoption->type = type;
 	newoption->lockmode = lockmode;
 
-	/*
-	 * Set the default lock mode for this option.  There is no actual way
-	 * for a module to enforce it when declaring a custom relation option,
-	 * so just use the highest level, which is safe for all cases.
-	 */
-	newoption->lockmode = AccessExclusiveLock;
-
 	MemoryContextSwitchTo(oldcxt);
 
 	return newoption;

src/test/modules/dummy_index_am/README

@@ -6,6 +6,7 @@ access method, whose code is kept a maximum simple.
 
 This includes tests for all relation option types:
 - boolean
+- enum
 - integer
 - real
 - strings (with and without NULL as default)

src/test/modules/dummy_index_am/expected/reloptions.out

@@ -20,6 +20,7 @@ CREATE INDEX dummy_test_idx ON dummy_test_tab
   option_bool = false,
   option_int = 5,
   option_real = 3.1,
+  option_enum = 'two',
   option_string_val = NULL,
   option_string_null = 'val');
 NOTICE:  new option value for string parameter null
@@ -32,9 +33,10 @@ SELECT unnest(reloptions) FROM pg_class WHERE relname = 'dummy_test_idx';
  option_bool=false
  option_int=5
  option_real=3.1
+ option_enum=two
  option_string_val=null
  option_string_null=val
-(5 rows)
+(6 rows)
 
 -- ALTER INDEX .. SET
 ALTER INDEX dummy_test_idx SET (option_int = 10);

src/test/modules/dummy_index_am/sql/reloptions.sql

@@ -20,6 +20,7 @@ CREATE INDEX dummy_test_idx ON dummy_test_tab
   option_bool = false,
   option_int = 5,
   option_real = 3.1,
+  option_enum = 'two',
   option_string_val = NULL,
   option_string_null = 'val');
 -- Silence again validation checks for strings until the end of the test.