config: always explicitly set PATH to something

config: always explicitly set PATH to something

In preparation for tightening up everything so that the
Perl scripts can run successfully under taint mode, always
explicitly configure a value for PATH.

Update the comments in Config.pm to reflect this (along with
more instructions showing how to keep the PATH in effect at
install time).

Update the FreezeConfig function to always provide a value
for PATH if one was not set and to always remove the other
environment variables that can cause taint failures (i.e.
IFS, CDPATH, ENV, BASH_ENV).

Update the get_girocco_config_var_list function in shlib.sh
to ensure it also has the new PATH semantics for cfg_path.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>