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.

Fix references to the no-longer-existent misc/ directory in comments, README files, and documentation