add the ability to convert (non-verbatim input) typed tabs to spaces

add the ability to convert (non-verbatim input) typed tabs to spaces
using the -E/--tabstospaces command line options, the "tabstospaces"
rcfile option, and the toggle Meta-E; also, change the short command
line option for --backupdir from -E to -C

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@2682 35c25a1d-7b9e-4130-9fde-d3aeb78583b8