bashrc: Fix PROMPT_COMMAND for gnome-terminal

Gnome-terminal sets PROMPT_COMMAND='__vte_prompt_command'. In
particular, it sets PROMPT_COMMAND without a terminal ';'. bashrc
assumes that if PROMPT_COMMAND has been previously set then it has
a terminal ';'.

To fix the bug, before inserting kw_prompt_command_old into
PROMPT_COMMAND, strip from kw_prompt_command_old any trailing ';' and
then append ';'.

Tags: public