bootstat: refine "Kernel panic - not syncing"

Provide some easy kernel panic subreasons mined from last kmesg,
generates a canonical boot reason (system boot reason) that
may aid triage.

Notably report kernel_panic,hung if [khungtaskd] triggers on
a livelock condition, forms a signals on the dashboards.

Helper function getSubreason modified to optionally enable checking
for a single quote resulting in a refactoring to ease maintenance of
the termination detection in the face of single bit errors heuristics.

Test: boot_reason_test.sh
Bug: 63736262
Bug: 33808187
Change-Id: I7fdd1e57e7a26095738175074306f0d2d59b1d69