Support a run-time configuration of the compression method to use for log files

[Julien-Elie]

People may want to use other compression methods than those hard-coded within m4/compress.m4 for their log files. We should provide a way to achieve that. For instance by adding native support in scanlogs and news.daily for a run-time configuration of the compression command to use. A new compressformat keyword might be used. Commands like compressformat="gzip -9" to improve compression with gzip, or compressformat="xz --memlimit=50%" to limit the xz compression hog (as well as adding support for xz) could then be used.

At the same time, we could replace the Z variable name with something like COMPRESS_EXT. Suggested by Christoph Biedl.

Also, we could document the already existing --with-log-compress configure flag in the relevant manual pages (at least INSTALL, newslog, news.daily and scanlogs. Current supported methods are gzip, bzip2, compress and cat (meaning no compression at all).