Fixed some compiler warnings identified by gcc's -Wextra option.

Fixed some compiler warnings identified by gcc's -Wextra option.

The following issues have been addressed:

 * comparison between signed and unsigned - this was found in several places
   throughout the code and has been fixed in various ways
 * missing initializer - an incomplete initializer has been used for two
   struct instances in perl.c
 * unused parameter - when applicable, the parameter has been removed; in
   thirteen cases the parameter is required by different library API's and in
   two cases the parameter was left in place to retain a consistent interface
   within the affected modules; as __attribute__((unused)) is a GNU extension,
   it has not been used to document those exceptions

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>