BIFScanner: Make filename->symbol transformation more robust

When trying to build bro from a path that contained a plus sign, an
invalid symbol name for the #ifdef guard was generated.
Do not change global event parameters in exec.bro

Addresses BIT-1294
Delete prebuilt python bytecode files from git.

BIT-1291 #close
Add/invoke "distclean" for testing directories.

BIT-1292 #close
Merge remote-tracking branch 'origin/topic/johanna/ticket-1294'

* origin/topic/johanna/ticket-1294:
  Do not change global event parameters in exec.bro

BIT-1294 #close
Merge branch 'master' of https://github.com/hillu/bro

* 'master' of https://github.com/hillu/bro:
  BIFScanner: Make filename->symbol transformation more robust
Update submodules/changes/version.