pivot: split on any whitespace

/proc/cmdline appends a '\n' at the end.  If the composefs= argument is
last on the commandline then we'll parse this \n as part of our
composefs= arg.  Splitting on any whitespace fixes this — the newline is
just another separator.

src/bin/composefs-pivot-sysroot.rs

@@ -10,7 +10,7 @@ fn parse_composefs_cmdline() -> Result<Sha256HashValue> {
     let mut proc_cmdline = std::fs::File::open("/proc/cmdline")?;
     proc_cmdline.read_to_end(&mut cmdline)?;
     // TODO?: officially we need to understand quoting with double-quotes...
-    for part in cmdline.split(|c| *c == b' ') {
+    for part in cmdline.split(|c| c.is_ascii_whitespace()) {
         if let Some(digest) = part.strip_prefix(b"composefs=") {
             let mut value = [0; 32];
             hex::decode_to_slice(digest, &mut value)?;