Move DirectoryExists() to common.c/h.
Add backend support for memory mapped channel blocks.

Once the feature is complete, this will allow external scripts or
programs to overlay data onto channel output data.  Currently this
can be used to test channels from the command line as well as turn
blocks of channels on or off.

A new 'fppmm' binary has been added as a client interface to the
memory mapped channel data file.  Currently this binary can be
used to:

  - turn test mode on or off
  - set individual channel values when in test mode
  - individually enable/disable overlay of mapped blocks' data
  - copy raw data from a specified file onto the channel output data

Theoretically the 'raw data' import feature could be used to
overlay a bitmap image onto a matrix if the channels were defined
and aligned in the right direction and order.  Currently it is left
up to the external process creating this raw data file to align the
channel data up properly if the display item has folded strings, or
other configurations which cause the channel data to be out of order
or non-contiguous.
Fix missing support to get info on a mapped channel block.

Fixes 'fpp -m MAP' output.
Add groundwork for documenting FPP config files.
Allow users to see and download files from the 'upload' directory.

If there is a failed upload attempt, the file may get stuck
here and we also plan on using this later for other features, so
add the ability to see what files are in the 'upload' directory
and download them for other use.

The plan is also to rework the download at some point to allow
downloading any uploaded file (sequences, media, etc.).
include memorymap.h for prototypes.
Fix USB DMX Open support and add a debug message to serial util.
Move GetE131LocalAddressFromInterface() and add a debug helper

Move GetE131LocalAddressFromInterface() from channeloutput/E131.c to
common.c as a more generic GetInterfaceAddress(char *interface) so we
can use it elsewhere.

Add HexDump(char *title, void *data, int len) which can dump a
block of data in hex and human readable format for debugging.