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

readline: Update initial readline commit -- make option configurable.  Add an interface to de-couple the readline implementation from NSH.  Misc. updates for coding style

Unix style TAB compleition, for builtin apps only

OS test: Sporadic test now emits a message if the max_repl configuration is too small for the test to work reliably

OS test: add some locking and unlocking of the scheduler to the sporadic scheduler test in order to test a few more cases

netutils: fix a typo while validating addrlen

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

NSH: Fix compile issue from all of the recent NETDB changes.  If NETDB is not enabled, we need to use inet_pton() instead of gethostbyname()

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/netutils/netlib and other files: Create netlib wrapper functions around dns_getserver() and dns_setserver() to isolate application code from changes to those interfaces.

If NSH or Webclient is selected, make sure that CONFIG_LIBC_NETDB is enabled; reanem dnsclient.h to dns.h

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.