Avoid strlen+xMalloc+xSnprintf, use xAsprintf instead

[BenBE]

There are several places that feature a length calculation, followed by an explicit xMalloc/xCalloc and a final xSnprintf into a buffer like this (example from PCPDynamicColumn_addMetric):

   size_t bytes = 16 + strlen(column->super.name);
   char* metricName = xMalloc(bytes);
   xSnprintf(metricName, bytes, "htop.column.%s", column->super.name);

This should better be implemented using xAsprintf:

   char* metricName = NULL;
   xAsprintf(&metricName, "htop.column.%s", column->super.name);

There are several more such places, in particular in the PCP platform code that I noticed (but likely elsewhere too).

[heysujal]

@BenBE I would like to work on this one.