Improve the code by static code analysis [2/3]: Performance

Improve the code by static code analysis [2/3]: Performance

Using the results cppcheck gave on the aMule source tree, the following fixes
and improvements were made:

- Use empty() for STL container emptyness checking. size() can take linear time
  while empty() is guaranteed to take constant time.
- Use prefix increment/decrement for non-primitive types (such as STL container
  iterators). A postfix operator usually needs a little extra code and
  housekeeping to keep the old value around.
- Remove unused variables. This includes variables that are assigned a value
  which is later never read.
- Pass non-POD type arguments by const reference instead of value.
- Catch by const reference instead of value.