Decode kevent structures logged via ktrace(2) in kdump.

• Add a new KTR_STRUCT_ARRAY ktrace record type which dumps an array of structures.
  
  The structure name in the record payload is preceded by a size_t containing the size of the individual structures. Use this to replace the previous code that dumped the kevent arrays dumped for kevent(). kdump is now able to decode the kevent structures rather than dumping their contents via a hexdump.
  
  One change from before is that the 'changes' and 'events' arrays are not marked with separate 'read' and 'write' annotations in kdump output. Instead, the first array is the 'changes' array, and the second array (only present if kevent doesn't fail with an error) is the 'events' array. For kevent(), empty arrays are denoted by an entry with an array containing zero entries rather than no record.

• Move kevent decoding tables from truss to libsysdecode.
  
  This adds three new functions to decode members of struct kevent: sysdecode_kevent_filter, sysdecode_kevent_flags, and sysdecode_kevent_fflags.
  
  kdump uses these helper functions to pretty-print kevent fields.

• Move structure definitions for freebsd11 and freebsd32 kevent structures to <sys/event.h> so that they can be shared with userland. The 32-bit structures are only exposed if _WANT_KEVENT32 is defined. The freebsd11 structures are only exposed if _WANT_FREEBSD11_KEVENT is defined. The 32-bit freebsd11 structure requires both.

• Decode freebsd11 kevent structures in truss for the compat11.kevent() system call.

• Log 32-bit kevent structures via ktrace for 32-bit compat kevent() system calls.

• While here, constify the 'void *data' argument to ktrstruct().

Reviewed by: kib (earlier version)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D12470