Enable Backtraces with --enable-debug

[ChrisJefferson]

This enables backtraces when GAP crashes, when GAP is compiled with --enable-debug. This will be (I hope!) particularly useful when GAP crashes on travis.

At the same time I made it so GAP will only print a backtrace when it actually crashes, not just because it was killed, or was terminated.

[coveralls]

Coverage decreased (-0.002%) to 84.643% when pulling 00a1020 on ChrisJefferson:backtrace into f56a68c on gap-system:master.

[codecov]

Codecov Report

Merging #3209 into master will decrease coverage by 8.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3209      +/-   ##
==========================================
- Coverage   84.76%   76.63%   -8.14%     
==========================================
  Files         687      681       -6     
  Lines      335865   326757    -9108     
==========================================
- Hits       284696   250397   -34299     
- Misses      51169    76360   +25191

Impacted Files	Coverage Δ
src/debug.c	29.41% <ø> (-7.44%)	⬇️
src/modules.h	16.66% <0%> (-83.34%)	⬇️
lib/ctbllatt.gi	4.94% <0%> (-79.62%)	⬇️
lib/ctblauto.gi	5.98% <0%> (-78.04%)	⬇️
lib/algliess.gi	5.95% <0%> (-72%)	⬇️
lib/proto.gi	13.4% <0%> (-71.14%)	⬇️
lib/ctblpope.gi	8% <0%> (-69.47%)	⬇️
lib/contfrac.gi	31.42% <0%> (-67.15%)	⬇️
lib/lierep.gi	23.58% <0%> (-62.55%)	⬇️
src/saveload.c	3.3% <0%> (-62.37%)	⬇️
... and 282 more

[fingolfin]

How does this interact with gdb or lldb?