Scripts: Fix exit status of xzgrep.

Omit the -q option from xz, gzip, and bzip2. With xz this shouldn't
matter. With gzip it's important because -q makes gzip replace SIGPIPE
with exit status 2. With bzip2 it's important because with -q bzip2
is completely silent if input is corrupt while other decompressors
still give an error message.

Avoiding exit status 2 from gzip is important because bzip2 uses
exit status 2 to indicate corrupt input. Before this commit xzgrep
didn't recognize corrupt .bz2 files because xzgrep was treating
exit status 2 as SIGPIPE for gzip compatibility.

zstd still needs -q because otherwise it is noisy in normal
operation.

The code to detect real SIGPIPE didn't check if the exit status
was due to a signal (>= 128) and so could ignore some other exit
status too.

src/scripts/xzgrep.in

@@ -156,11 +156,11 @@
 
 for i; do
   case $i in
-    *[-.][zZ] | *_z | *[-.]gz | *.t[ag]z) uncompress="gzip -cdfq";;
-    *[-.]bz2 | *[-.]tbz | *.tbz2) uncompress="bzip2 -cdfq";;
-    *[-.]lzo | *[-.]tzo) uncompress="lzop -cdfq";;
-    *[-.]zst | *[-.]tzst) uncompress="zstd -cdfq";;
-    *) uncompress="$xz -cdfq";;
+    *[-.][zZ] | *_z | *[-.]gz | *.t[ag]z) uncompress="gzip -cdf";;
+    *[-.]bz2 | *[-.]tbz | *.tbz2) uncompress="bzip2 -cdf";;
+    *[-.]lzo | *[-.]tzo) uncompress="lzop -cdf";;
+    *[-.]zst | *[-.]tzst) uncompress="zstd -cdfq";; # zstd needs -q.
+    *) uncompress="$xz -cdf";;
   esac
   # Fail if xz or grep (or sed) fails.
   xz_status=$(
@@ -205,8 +205,14 @@
   # fail occurred previously, nothing worse can happen
   test $res -gt 1 && continue
 
-  test "$xz_status" -eq 0 || test "$xz_status" -eq 2 \
-      || test "$(kill -l "$xz_status" 2> /dev/null)" = "PIPE" || r=2
+  if test "$xz_status" -eq 0; then
+    :
+  elif test "$xz_status" -ge 128 \
+      && test "$(kill -l "$xz_status" 2> /dev/null)" = "PIPE"; then
+    :
+  else
+    r=2
+  fi
 
   # still no match
   test $r -eq 1 && continue