* misc.c (compress_algo_to_string, string_to_compress_algo,
check_compress_algo): Add bzip2.
* compress.c (compress_filter): Make static to help force the use of
push_compress_filter.  Remove default algorithm setting since that is done
in push_compress_filter now.
* main.h: Use named algorithm.
* filter.h, compress.c (push_compress_filter, push_compress_filter2): New.
Figure out which is the appropriate compression filter to use, and push it
into place.
* compress.c (handle_compressed), encode.c (encode_simple, encode_crypt),
sign.c (sign_file, sign_symencrypt_file), import.c (read_block), export.c
(do_export): Use push_compress_filter instead of pushing the compression
filter ourselves.
* compress-bz2.c: New.  Bzlib versions of the compression filter routines.
* Makefile.am: Include compress-bz2.c if bz2lib is available.