This commit sends a flag indicating the presence of the "--bypass"
option from the donor node to the joiner nodes during rsync IST,
because without such a flag it is impossible to distinguish IST
from the SST on the joiner nodes (in IST/SST scripts, because the
"--bypass" option is still not passed to scripts from server code).
Specifically, this fixes an issue with binary logs disappearing
after IST (via rsync). There are also changes to diagnostic messages
here that will make it easier to diagnose script-related problems
in the future when debugging and when checking the logs. This commit
also adds more robust signal handlers - to handle exceptions during
script execution. These handlers won't mask some crashes and it
also unifies exit codes between different scripts. These changes
have already been helpful to debugging "bypass" flag handling.