daemon: Add gzip log compression.

* nix/nix-daemon/guix-daemon.cc (GUIX_OPT_LOG_COMPRESSION): New macro.
(options): Mark "disable-log-compression" as hidden and add
"log-compression".
(parse_opt): Handle GUIX_OPT_LOG_COMPRESSION.
* nix/libstore/build.cc (DerivationGoal): Add 'gzLogFile'.
(openLogFile): Initialize it when 'logCompression' is COMPRESSION_GZIP.
(closeLogFile, handleChildOutput): Honor 'gzLogFile'.
* nix/libstore/globals.hh (Settings)[compressLog]: Remove.
[logCompression]: New field.
(CompressionType): New enum.
* nix/libstore/globals.cc (Settings::Settings): Initialize it.
(update): Remove '_get' call for 'compressLog'.
* nix/local.mk (guix_daemon_LDADD, guix_register_LDADD): Add -lz.
* guix/store.scm (log-file): Handle '.gz' log files.
* tests/guix-daemon.sh: Add test with '--log-compression=gzip'.
* doc/guix.texi (Invoking guix-daemon): Adjust accordingly.
* config-daemon.ac: Check for libz and zlib.h.