sd-bus: use terms from the D-Bus Specification a bit more

D-Bus' type hierarchy as described in the spec is:

\- basic
   \- fixed type (u, i, etc.)
   \- string-like type (s, o, g)
\- container

Someone seems to have referred to basic types as "simple types" at
some point, but that term isn't defined in the D-Bus Specification,
and seems redundant.

So far I haven't renamed functions that use "trivial" in their names
to mean "fixed type", to avoid confusion about whether a struct of
constant length, like (iu), is a fixed type. The answer is that it is
fixed-length, but is not a "fixed type", so I can see that something
like bus_type_is_fixed() might be ambiguous.

src/libsystemd/sd-bus/PORTING-DBUS1

@@ -156,11 +156,11 @@ multiple items. Some restrictions apply however:
    contained in the payload, as well is immediately before framing of a
    Gvariant, as well after as any padding bytes if there are any. The
    padding bytes must be wholly contained in the preceding
-   PAYLOAD_VEC/PAYLOAD_MEMFD item. You may not split up simple types
-   nor arrays of trivial types. The latter is necessary to allow APIs
-   to return direct pointers to linear chunks of fixed size trivial
-   arrays. Examples: The simple types "u", "s", "t" have to be in the
-   same payload item. The array of simple types "ay", "ai" have to be
+   PAYLOAD_VEC/PAYLOAD_MEMFD item. You may not split up basic types
+   nor arrays of fixed types. The latter is necessary to allow APIs
+   to return direct pointers to linear arrays of numeric
+   values. Examples: The basic types "u", "s", "t" have to be in the
+   same payload item. The array of fixed types "ay", "ai" have to be
    fully in contained in the same payload item. For an array "as" or
    "a(si)" the only restriction however is to keep each string
    individually in an uninterrupted item, to keep the framing of each

src/libsystemd/sd-bus/bus-type.h

@@ -29,6 +29,8 @@
 bool bus_type_is_valid(char c) _const_;
 bool bus_type_is_valid_in_signature(char c) _const_;
 bool bus_type_is_basic(char c) _const_;
+/* "trivial" is systemd's term for what the D-Bus Specification calls
+ * a "fixed type": that is, a basic type of fixed length */
 bool bus_type_is_trivial(char c) _const_;
 bool bus_type_is_container(char c) _const_;