When executing commands over SSH, such as SCP, the server (tput)
complains that there is no terminal defined. This fix sets the T_*
variables only if the TERM variable is not empty.
Add a function add_to_path so that it can prepend any specified path
to the PATH environment variable, but only if it is not already there.
Don't worry about (re)setting the LS_COLORS environment if it's already
been set by the system bashrc.