Fixed A, -B, and -S options, thanged @ to ~, fixed a bug with negative chars.

Before this patcH -B was always active, -A & -S had no effect.
Only include prosigns if -B is specified, per the documentation.
Added '@' character (.--.-.), in use if -B is enabled.
Changed representation of <SN> prosign from '@' to '~' to accommodate '@'.
Don't crash if char is signed and a negative value is sent.

Signed-off-by: Eric S. Raymond <esr@thyrsus.com>