compress: consolidate all compression into compress.c with dlopen

Move the push-based streaming compression API from import-compress.c
into compress.c and delete import-compress.c/h. This consolidates all
compression code in one place and makes all compression libraries
(liblzma, liblz4, libzstd, libz, libbz2) runtime-loaded via dlopen
instead of directly linked.

Introduce opaque Compressor/Decompressor types backed by a heap-
allocated struct defined only in compress.c, keeping all third-party
library headers out of compress.h.

Rewrite the per-codec fd-to-fd stream functions as thin wrappers around
the push API via generic compress_stream()/decompress_stream() taking a
Compression type parameter. Integrate LZ4 into this framework using the
LZ4 Frame API, eliminating all LZ4 special-casing.

Extend the Compression enum with COMPRESSION_GZIP and COMPRESSION_BZIP2
and add the corresponding blob, startswith, and stream functions for
both.

Rename the ImportCompress types and functions: ImportCompressType becomes
the existing Compression enum, ImportCompress becomes Compressor (with
Decompressor typedef), and all import_compress_*/import_uncompress_*
become compressor_*/decompressor_*. Rename dlopen_lzma() to dlopen_xz()
for consistency. Make compression_to_string() return lowercase by
default.

Add INT_MAX/UINT_MAX overflow checks for LZ4, zlib, and bzip2 blob
functions where the codec API uses narrower integer types than our
uint64_t parameters.

Migrate test-compress.c and test-compress-benchmark.c to the TEST()
macro framework, new assertion macros, and codec-generic loops instead
of per-codec duplication.

Co-developed-by: Claude Opus 4.6 <noreply@anthropic.com>

test/units/TEST-04-JOURNAL.SYSTEMD_JOURNAL_COMPRESS.sh

@@ -17,6 +17,13 @@ EOF
 systemctl reset-failed systemd-journald.service
 
 for c in NONE XZ LZ4 ZSTD; do
+    # compression_to_string() returns "uncompressed" for COMPRESSION_NONE
+    if [[ "${c}" == NONE ]]; then
+        log_name="uncompressed"
+    else
+        log_name="${c,,}"
+    fi
+
     cat >/run/systemd/system/systemd-journald.service.d/compress.conf <<EOF
 [Service]
 Environment=SYSTEMD_JOURNAL_COMPRESS=${c}
@@ -28,14 +35,20 @@ EOF
     ID="$(systemd-id128 new)"
     systemd-cat -t "$ID" bash -c "for ((i=0;i<100;i++)); do echo -n hoge with ${c}; done; echo"
     journalctl --sync
-    timeout 10 bash -c "until SYSTEMD_LOG_LEVEL=debug journalctl --verify --quiet --file /var/log/journal/$MACHINE_ID/system.journal 2>&1 | grep -F 'compress=${c}' >/dev/null; do sleep .5; done"
+    timeout 10 bash -c "until SYSTEMD_LOG_LEVEL=debug journalctl --verify --quiet --file /var/log/journal/$MACHINE_ID/system.journal 2>&1 | grep -F 'compress=${log_name}' >/dev/null; do sleep .5; done"
 
     # $SYSTEMD_JOURNAL_COMPRESS= also works for journal-remote
     if [[ -x /usr/lib/systemd/systemd-journal-remote ]]; then
         for cc in NONE XZ LZ4 ZSTD; do
+            if [[ "${cc}" == NONE ]]; then
+                cc_log_name="uncompressed"
+            else
+                cc_log_name="${cc,,}"
+            fi
+
             rm -f /tmp/foo.journal
             SYSTEMD_JOURNAL_COMPRESS="${cc}" /usr/lib/systemd/systemd-journal-remote --split-mode=none -o /tmp/foo.journal --getter="journalctl -b -o export -t $ID"
-            SYSTEMD_LOG_LEVEL=debug journalctl --verify --quiet --file /tmp/foo.journal 2>&1 | grep -F "compress=${cc}" >/dev/null
+            SYSTEMD_LOG_LEVEL=debug journalctl --verify --quiet --file /tmp/foo.journal 2>&1 | grep -F "compress=${cc_log_name}" >/dev/null
             journalctl -t "$ID" -o cat --file /tmp/foo.journal | grep -F "hoge with ${c}" >/dev/null
         done
     fi