More options for color in pdksh

pdksh uses an arbitrary, non-printing character to delimit color escape sequences, but no one character works well in all operating systems and on all terminals. This update

• Uses \021 instead of \016 for better support of NetBSD and OpenBSD (props @scuzzilla)
• Allows the user to specify their own non-printing character by setting the environment variable POLYGLOT_NP
• Allow the user to turn of color for pdksh by setting POLYGLOT_PDKSH_COLORS=0