Fixed various signedness issues identified by -Wextra.

Fixed various signedness issues identified by -Wextra.

The following two issues have been addressed:

 * comparison between signed and unsigned - this was found in several places
   throughout the code and has been fixed by switching to more appropriate
   types or adding appropriate explicit casts.
 * comparison of unsigned expression < 0 is always false - this was found in
   the processes and vserver plugins where a size_t had wrongly been used
   instead of a ssize_t and an int respectively.