Modbus Master is missing many files and doesn't compile at all. I'm attaching diff with my implementation. More details in https://groups.yahoo.com/neo/groups/nuttx/conversations/topics/13734

apps/nshlib: nsh_getdirpath(), use snprint instead of sprintf to avoid possibility of buffer overrun.  Noted by Chung Hwan Kim.

Add a new buttons example that uses the button character driver instead of the architecture buttons directly

apps/nshlib: Add printf command to NSH, e.g., controlling /dev/userleds from command line: nsh> printf \x01 > /dev/userleds

The previous commit was reverted.  My concern was misplaced:  There is a guard boolean to prevent running the init script more than once.  This comment just restores some of the good cosmetic changes that were included in the reverted commit.

Revert "This commit adds nsh_consolechild().  I noticed that there are several examples that call nsh_consolemain.c to create to create a new NSH session.  That will not always work because nsh_consolemain() performs the one-time initialization needed by the NSH library:  Mounting, reading, and running the start-up script, configuring USB tracing.  Some of those actions will cause failures if attempted twice."

This reverts commit 4d364d83.