Move lib/stdio/lib_fgets.c to apps/system/readline; simplify fgets(); use...

Move lib/stdio/lib_fgets.c to apps/system/readline; simplify fgets(); use readline instead of fgets in NSH

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@4356 42af7a65-404d-4744-a932-0658087f49c3