nshlib: Now that NuttX automatically converts block to character devices, the implemention of the DD command no longer has to muck with block to character conversion

Macros PR_BEGIN_EXTERN_C and PR_END_EXTERN_C were not defined in all contexts.  Remove definition and replace with explicit expansion.  From Stefan Kolb

apps/nshlib: Fix error handling in 'mv' command.  On a failure to expand the second path, the memory allocated for the expansion of the first path was not being freed.  From Bruno Herrera.

apps/nshlib: Fix error handling in 'cat' command.  On a failure to allocate memory, a file was not being closed.  From Bruno Herrera.

apps/system/netdb would not build unless CONFIG_NETDB_HOSTFILE was defined because it needed gethostbyname.  Noted by OrbitalFox

apps/examples/usbserial: Can now be run as an NSH builtin-function. Now uses a configurable IO buffer size

apps/tools/mkkconfig.sh:  The top-level Kconfig file is not auto-generated.  The autogenerated Kconfig file will be constructed so that every second level directory that contains a Kconfig file will automatically be sourced

readline():  A a configuration option to enable/disable command line history; Additional cosmetic changes from code review

readline/NSH: Extend the tab-completion logic so that NSH commands can also be completed by pressing the tab key

Adjust for increased size of the scheduling policy field from 1 to 2 bits to allow additional, planned scheduling policies

apps/examples/nxlines:  Clear a slightly larger rectangle than the one that we drew.  This eliminates edge effects due to applying anti-aliasing twice:  Once when the line was drawn, and again when the line was erased by overwriting it with the background color

apps/netuils/dnsclient and include/netutils/dnsclient.h:  Move the DNS client logic into the NuttX C library.  It is a necessary part for the full implementation of the netdb logic and provides more flexibility in that location.