Fixes bug 9882.

The cleanup code will try to flush the output buffer in some
circumstances, which is not valid if we're handling an async signal
(since it might have interrupted some partial I/O in the main thread).
These signals now set a flag and try to let the main I/O handler take
care of the exit strategy.  Fixes a protocol error that could happen
when trying to exit after a kill signal.

- If iconv() returns EINVAL or EILSEQ and the error is being ignored, make
  sure that there is room in the output buffer to store the erroneous char.
- When accepting an erroneous char, be sure to break if there are no more
  input characters (without calling iconv() with a zero input length).

- Avoid relinking a file that is already linked correctly.
- Avoid trashing the stat buffer of an existing file in try_dests_reg().

when the dest file is unlinked and the hard link fails.

Fixes bug #5644.

Fixes bug 9212.

When a daemon is sent multiple request args, they are now joined into a
single return value (separated by spaces) so that the RSYNC_REQUEST
environment variable is accurate for any "pre-xfer exec".  The values
in RSYNC_ARG# vars are no longer truncated at the "." arg, so that all
the request values are also listed (separately) in RSYNC_ARG#.

Fixes debian bug 624826.

Fixes bug 8565.

Fixes bug 7965.

Fixes bug 8036.

The sender no longer allows a filelist to be sent in the middle of
parsing an incoming message, so that the directory sending doesn't block
all further input reading.  The generator no longer allows recursive
reading of info/error messages when it is waiting for the message buffer
to flush.  This avoids a stack overflow when lots of messages are coming
from the receiver and the sender is not reading things quickly enough.
The I/O code now avoids sending debug messages that could mess up the
I/O buffer it was in the middle of tweaking.  This fixes an infinite
loop in reduce_iobuf_size() with high levels of debug enabled.  Several
I/O-related messages were changed to output only when --msgs2stderr is
enabled.

Fixes bug 8665.