Improve hash_create()'s API for some added robustness.

Invent a new flag bit HASH_STRINGS to specify C-string hashing, which
was formerly the default; and add assertions insisting that exactly
one of the bits HASH_STRINGS, HASH_BLOBS, and HASH_FUNCTION be set.
This is in hopes of preventing recurrences of the type of oversight
fixed in commit a1b8aa1e4 (i.e., mistakenly omitting HASH_BLOBS).

Also, when HASH_STRINGS is specified, insist that the keysize be
more than 8 bytes.  This is a heuristic, but it should catch
accidental use of HASH_STRINGS for integer or pointer keys.
(Nearly all existing use-cases set the keysize to NAMEDATALEN or
more, so there's little reason to think this restriction should
be problematic.)

Tweak hash_create() to insist that the HASH_ELEM flag be set, and
remove the defaults it had for keysize and entrysize.  Since those
defaults were undocumented and basically useless, no callers
omitted HASH_ELEM anyway.

Also, remove memset's zeroing the HASHCTL parameter struct from
those callers that had one.  This has never been really necessary,
and while it wasn't a bad coding convention it was confusing that
some callers did it and some did not.  We might as well save a few
cycles by standardizing on "not".

Also improve the documentation for hash_create().

In passing, improve reinit.c's usage of a hash table by storing
the key as a binary Oid rather than a string; and, since that's
a temporary hash table, allocate it in CurrentMemoryContext for
neatness.

Discussion: https://postgr.es/m/590625.1607878171@sss.pgh.pa.us

src/backend/storage/ipc/shmem.c

@@ -292,7 +292,6 @@ void
 InitShmemIndex(void)
 {
 	HASHCTL		info;
-	int			hash_flags;
 
 	/*
 	 * Create the shared memory shmem index.
@@ -304,11 +303,11 @@ InitShmemIndex(void)
 	 */
 	info.keysize = SHMEM_INDEX_KEYSIZE;
 	info.entrysize = sizeof(ShmemIndexEnt);
-	hash_flags = HASH_ELEM;
 
 	ShmemIndex = ShmemInitHash("ShmemIndex",
 							   SHMEM_INDEX_SIZE, SHMEM_INDEX_SIZE,
-							   &info, hash_flags);
+							   &info,
+							   HASH_ELEM | HASH_STRINGS);
 }
 
 /*
@@ -329,6 +328,11 @@ InitShmemIndex(void)
  * whose maximum size is certain, this should be equal to max_size; that
  * ensures that no run-time out-of-shared-memory failures can occur.
  *
+ * *infoP and hash_flags must specify at least the entry sizes and key
+ * comparison semantics (see hash_create()).  Flag bits and values specific
+ * to shared-memory hash tables are added here, except that callers may
+ * choose to specify HASH_PARTITION and/or HASH_FIXED_SIZE.
+ *
  * Note: before Postgres 9.0, this function returned NULL for some failure
  * cases.  Now, it always throws error instead, so callers need not check
  * for NULL.