Change safe_read() to select() before reading.
Fix timeout checking in safe_read().
Avoid useless keepalive msgs that would kill an older rsync.

This fix avoids the sending of keep-alive messages from the receiver
to the sender when we are still sending the file list (at which time
an older rsync would die if it received such a keep-alive message).
The messages aren't actually needed, since we haven't forked yet, and
the single flow of data keeps the procs alive.
Fix itemize bug with --link-dest, -X, and -n.

When running with --*-dest & -X, some alt-dest-found files would not
use the right name when looking up old attrs in itemize(), causing a
weird error for a --dry-run copy.  Fixes bug 10238.
Use chunked xattr reading in OS X sys_lgetxattr().
Create and use write_bigbuf() function for extra-large buffer sizes.