Convert filterArgs.sh to POSIX shell

requested by qlal1 in the #!/bin/mksh channel on Freenode IRC

Author: mirabilos

contrib/filterArgs.sh

@@ -4,7 +4,7 @@
 # Loop over all command line arguments
 for i in "$@"; do
     # If an argument starts with -L, echo it out sans -L!
-    if [[ $i == -L* ]]; then
-        echo "\"${i:2:${#i}}\""
-    fi
+    case $i in
+    -L*) printf '"%s"\n' "${i#-L}" ;;
+    esac
 done