Add debug tracing support

Add thud_trace.sh - a module managing tracing with a DEBUG trap.
Add requisite thud_traps.sh - a module managing trap set state.
Separate enabling/disabling of strict mode

Do enabling/disabling of strict mode in dedicated functions
(thud_strict_on/off), instead of the thud_strict_set.
This makes it somewhat easier to read and modify.
Convert README to README.md