Should've probably been two commits, but:
1. Cleaned up tracebd.py's header generation to be consistent with
dbgbmap.py and other scripts.
Percentage fields are now consistently floats in all scripts,
allowing user-specified precision when punescaping.
2. Moved header generation up to where we still have the disk open (in
dbgbmap[d3].py), to avoid issues with lazy Lfs attrs trying to access
the disk after it's been closed.
Found while testing with --title='cksum %(cksum)08x'. Lfs tries to
validate the gcksum last minute and things break.
Christopher Haster
50f652d44f