Add explicit --pass-through flag and default to enabled for *cat (#3223)

Fixes #3211.

Adds the `--[no-]pass-through` flag which enables/disables pass-through mode.

* `zstdcat`, `zcat`, and `gzcat` default to `--pass-through`.
  Pass-through mode can be disabled by passing `--no-pass-through`.
* All other binaries default to not setting pass-through mode.
  However, we preserve the legacy behavior of enabling pass-through
  mode when writing to stdout with `-f` set, unless pass-through
  mode is explicitly disabled with `--no-pass-through`.

Adds a new test for this behavior that should codify the behavior we want.

tests/cli-tests/decompression/pass-through.sh

@@ -0,0 +1,57 @@
+#!/bin/sh
+
+set -e
+
+. "$COMMON/platform.sh"
+
+echo "" > 1
+echo "2" > 2
+echo "23" > 3
+echo "234" > 4
+echo "some data" > file
+
+println "+ passthrough enabled"
+
+zstd file
+
+# Test short files
+zstd -dc --pass-through 1 2 3 4
+
+# Test *cat symlinks
+zstdcat file
+"$ZSTD_SYMLINK_DIR/zcat" file
+"$ZSTD_SYMLINK_DIR/gzcat" file
+
+# Test multiple files with mix of compressed & not
+zstdcat file file.zst
+zstdcat file.zst file
+
+# Test --pass-through
+zstd -dc --pass-through file
+zstd -d --pass-through file -o pass-through-file
+
+# Test legacy implicit passthrough with -fc
+zstd -dcf file
+zstd -dcf file file.zst
+zstd -df < file
+zstd -dcf < file file.zst -
+zstd -dcf < file.zst file -
+
+$DIFF file pass-through-file
+
+println "+ passthrough disabled"
+
+# Test *cat
+zstdcat --no-pass-through file && die "should fail"
+"$ZSTD_SYMLINK_DIR/zcat" --no-pass-through file && die "should fail"
+"$ZSTD_SYMLINK_DIR/gzcat" --no-pass-through file && die "should fail"
+# Test zstd without implicit passthrough
+zstd -d file -o no-pass-through-file && die "should fail"
+zstd -d < file && die "should fail"
+
+# Test legacy implicit passthrough with -fc
+zstd --no-pass-through -dcf file && die "should fail"
+zstd --no-pass-through -dcf file file.zst && die "should fail"
+zstd --no-pass-through -df < file && die "should fail"
+zstd --no-pass-through -dcf < file file.zst - && die "should fail"
+zstd --no-pass-through -dcf < file.zst file - && die "should fail" ||:

tests/cli-tests/decompression/pass-through.sh.stderr.exact

@@ -0,0 +1,10 @@
+zstd: file: unsupported format 
+zstd: file: unsupported format 
+zstd: file: unsupported format 
+zstd: file: unsupported format 
+zstd: /*stdin*\: unsupported format 
+zstd: file: unsupported format 
+zstd: file: unsupported format 
+zstd: /*stdin*\: unsupported format 
+zstd: /*stdin*\: unsupported format 
+zstd: file: unsupported format 

tests/cli-tests/decompression/pass-through.sh.stdout.exact

@@ -0,0 +1,25 @@
++ passthrough enabled
+
+2
+23
+234
+some data
+some data
+some data
+some data
+some data
+some data
+some data
+some data
+some data
+some data
+some data
+some data
+some data
+some data
+some data
+some data
++ passthrough disabled
+some data
+some data
+some data