Add an arg-protection idiom using backslash-escapes

Add an arg-protection idiom using backslash-escapes

The new default is to protect args and options from unintended shell
interpretation using backslash escapes.  See the new `--old-args` option
for a way to get the old-style splitting.  This idiom was chosen over
making `--protect-args` enabled by default because it is more backward
compatible (e.g. it works with rrsync). Fixes #272.