gdb: rename unwindonsignal to unwind-on-signal

gdb: rename unwindonsignal to unwind-on-signal

We now have unwind-on-timeout and unwind-on-terminating-exception, and
then the odd one out unwindonsignal.

I'm not a great fan of these squashed together command names, so in
this commit I propose renaming this to unwind-on-signal.

Obviously I've added the hidden alias unwindonsignal so any existing
GDB scripts will keep working.

There's one test that I've extended to test the alias works, but in
most of the other test scripts I've changed over to use the new name.

The docs are updated to reference the new name.