Separated the "expr --unwind-on-error" behavior into two parts, actual errors (i.e. crashes) which continue to be
controlled by the --unwind-on-error flag, and --ignore-breakpoint which separately controls behavior when a called
function hits a breakpoint.  For breakpoints, we don't unwind, we either stop, or ignore the breakpoint, which makes
more sense.
Also make both these behaviors globally settable through "settings set".
Also handle the case where a breakpoint command calls code that ends up re-hitting the breakpoint.  We were recursing
and crashing.  Now we just stop without calling the second command.

<rdar://problem/12986644>
<rdar://problem/9119325>