Various fixes and improvements to printpcs

Search for an addr2line that supports elf32-i386.  Don't print the
path of addr2line as a side-effect of which.  Don't uselessly pipe
"grep '^ '" to addr2line's stdin.  Enable as many pretty-printing and
otherwise helpful options as possible (this makes the output *much*
more pleasant on modern addr2lines).

printpcs

@@ -1,11 +1,14 @@
 #!/bin/sh
 
-# Decode the symbols from a panic stack trace on stdin
+# Decode the symbols from a panic EIP list
 
-if which addr2line
-then
-    p="addr2line"
-else
-    p="i386-jos-elf-addr2line"
-fi
-echo grep '^ ' | $p -e kernel $*
+# Find a working addr2line
+for p in i386-jos-elf-addr2line addr2line; do
+    if which $p 2>&1 >/dev/null && \
+       $p -h 2>&1 | grep -q '\belf32-i386\b'; then
+        break
+    fi
+done
+
+# Enable as much pretty-printing as this addr2line can do
+$p $($p -h | grep ' -[aipsf] ' | awk '{print $1}') -e kernel "$@"