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.

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.

Here are some offenders: examples/cc3000/shell.c, examples/nsh/nsh_main.c, examples/nxterm/nxterm_main.c, and examples/pty_test/pty_test.c.  It is possible that for some of those, that represents the first call to nsh_consolemain().  But probably not.

To give other initialization options, I added nsh_consolechild() which is identical to nsh_consolemain() other than it omits all of the one-time initialization.  Of course, you would not get the result that you wanted if nsh_consolechild() were called without nsh_consolemain().

apps/examples/system:  Add a simple test of the system command.  Fix the first bug discovered by the test.

apps/system/system:  Add a generic system command.  Current implentation cannot use /bin/sh and spawns the custom NSH system command directly.

apps/nshlib/nsh_syscmds.c: missing semicolon

apps/examples/oneshot:  If the requested delay is > max_delay, then break the delay up into several pieces.