Add support for lz4 write filter.
Fix build failure with lz4 r120 and later.
Fix build failure on FreeBSD 8.x.
Display filename encoding test names respectively to know what encoding test
was successful, failed or skipped.
Eliminate dead assignment.
Avoid an analyzer error "Dereference of null pointer",
which will not happen but analyzer claims.
Add lz4 filter entry to archive_read_append_filter function.
Update xxhash.c to lz4 r120 version;
o remove xxhash.h, which is unneeded in libarchive and causes
  build failure for function definitions.
o convert single comment statements to block comment statements
  because some C compiler cannot handle it.
Improve lha decompression performance.
Document lz4 compression, lzop compression and other filters
which were not documented in archive_read_filter.3 or
archive_write_filter.3.