libsensors: Get rid of arbitrary limit on per-type sensor count

When gathering the attributes of each hwmon chip, libsensors uses a
temporary structure in memory to order and group all the attributes
into features. This temporary structure used to be a single array with
room for every possible attribute/subfeature. While simple, this
approach required to predefine a maximum number of per-type sensor
that could be handled.

In order to get rid of this arbitrary limit, which we hit and had to
raise three times already, I changed the temporary structure to an
array of dynamically allocated per-type subattribute arrays. This lets
us not allocate any memory for types which aren't implemented by a
given chip, and more importantly, this lets us reallocate room for
more attributes of a given type as needed.

I decided to allocate chunks of 8 attributes at a time, as this seemed
a good compromise between two frequent reallocations and
over-provisioning. It could be tweaked if needed.

Icing on the cake, I benchmarked this change on two different systems
and it results in performance gains. The total heap usage as reported
by valgrind is down by 50% on average, with peak memory consumption
(as reported by valgrind's massif) also down by 43% on average. The
total instructions count (as reported by valgrind's callgrind) is down
by 11% on average, with measured execution time also down by a few
percents. Valgrind rocks, BTW.

I have some ideas to optimize the memory allocations further, but I do
not expect such a huge gain from them. They may not even improve peak
memory consumption as massif shows the peak is somewhere else now at
least in some cases.

doc/libsensors-API.txt

@@ -6,6 +6,10 @@ over time. This document summarizes these evolutions so that application
 authors can quickly figure out how to test for the availability of a
 given new feature.
 
+0x440	lm-sensors 3.4.0
+* Defined SENSORS_FEATURE_MAX
+  enum sensors_feature_type SENSORS_FEATURE_MAX
+
 0x432	lm-sensors 3.3.2 to 3.3.5
 * Added support for new sysfs attributes
   enum sensors_subfeature_type SENSORS_SUBFEATURE_IN_AVERAGE
@@ -19,6 +23,7 @@ given new feature.
   enum sensors_subfeature_type SENSORS_SUBFEATURE_FAN_MAX
   enum sensors_subfeature_type SENSORS_SUBFEATURE_FAN_MIN_ALARM
   enum sensors_subfeature_type SENSORS_SUBFEATURE_FAN_MAX_ALARM
+
 0x431	lm-sensors 3.3.0 to 3.3.1
 * Added support for intrusion detection
   enum sensors_feature_type SENSORS_FEATURE_INTRUSION