Removed some unused parameters; added SAL_UNUSED_PARAMETER.

SAL_UNUSED_PARAMETER (expanding to __attribute__ ((unused)) for GCC)
is used to annotate legitimately unused parameters, so that static
analysis tools can tell legitimately unused parameters from truly
unnecessary ones.  To that end, some patches for external modules
are also added, that are only applied when compiling with GCC and
add necessary __attribute__ ((unused)) in headers.