check for repeated java backtraces

Repeated Java backtraces will not be printed. On the first backtrace
that differs from the previously cached trace, a repeat message will be
printed, and the current trace will be output.

For clarity: the "DVM:BT_REPEAT:X" log message indicates that the
_previous_ java backtrace was repeated X times.

Signed-off-by: Jeremy C. Andrus <jeremya@cs.columbia.edu>
suppress full Java method signature by default

They can be re-enabled by setting the DVM_BT_GET_SIGNATURE preprocessor
token to "true"

Signed-off-by: Jeremy C. Andrus <jeremya@cs.columbia.edu>
increase backtrace speed (slightly)

This patch is more prepatory than beneficial - it removes some cruft,
but doesn't change the tracing speed by all that much.

NOTE: The log format has changed slightly. There is no longer an ending
line for the backtrace; the BT:START: line now contains the number of
subsequent lines in the trace.

Signed-off-by: Jeremy C. Andrus <jeremya@cs.columbia.edu>