Add many experimental enhancements for the visual debugger

- V@ is an alias to set cmd.vprompt
- New config variables!
  stack.size = size of stack displayed in Vpp
  stack.delta = substract that delta to the stack address
  stack.anotated = use pxa instead of pxw
  scr.highlight = word to highlight by RCons
  hex.flagsz = override flag size in pxa
- Fix x86 udis anal plugin size of invalid instructions
- Fix 'hud file not found' issue
- Add 'Vdh' to set scr.highlight
- Fix V: wheel scroll issue
- Realign pxa marks if addr > UT32_MAX
- Set hex.flagsz = 9999 to cover the whole area instead of just 1 byte
Fix crash in RCons.highlight
More highlight fixes
Fix labels (local flags)
fix unused var warning in bin_elf.c
Add anal.null, a new fallback analysis plugin
Define and use R_NOTNULL in RBinPlugin.load_bytes
Fix pxa trash chars
Fix #1393 and update other pf help messages