Clean up the port

• Use = instead of += when unnecessary when assigning values to standard ports variables
• Set to various option-specific variables via OPTION_VARIABLEs.
• Improve the description of USERPREFS; it's better to have a more descriptive option instead of defining a custom warning in "pre-everything::".
• Use OPTION_CONFIGURE_ENABLE instead of OPTION_CONFIGURE_ON.
• Install manual pages at the preferred directory
• Install *.sample in ETCDIR with @sample, update pkg-message accordingly.