Reject huge_pages=on if shared_memory_type=sysv.

It doesn't work (it could, but hasn't been implemented).
Back-patch to 12, where shared_memory_type arrived.
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/163271880203.22789.1125998876173795966@wrigleys.postgresql.org

doc/src/sgml/config.sgml

@@ -1664,7 +1664,9 @@ include_dir 'conf.d'
        <para>
         At present, this setting is supported only on Linux and Windows. The
         setting is ignored on other systems when set to
-        <literal>try</literal>.
+        <literal>try</literal>.  On Linux, it is only supported when
+        <varname>shared_memory_type</varname> is set to <literal>mmap</literal>
+        (the default).
        </para>
 
        <para>

src/backend/port/sysv_shmem.c

@@ -686,6 +686,12 @@ PGSharedMemoryCreate(Size size,
 				 errmsg("huge pages not supported on this platform")));
 #endif
 
+	/* For now, we don't support huge pages in SysV memory */
+	if (huge_pages == HUGE_PAGES_ON && shared_memory_type != SHMEM_TYPE_MMAP)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("huge pages not supported with the current shared_memory_type setting")));
+
 	/* Room for a header? */
 	Assert(size > MAXALIGN(sizeof(PGShmemHeader)));