move the main terminal initialization functions, aside from initscr(),

move the main terminal initialization functions, aside from initscr(),
into a new terminal_init() function, and convert nano to use it; also,
when reloading the newly spell-checked temporary file in
do_alt_speller(), call terminal_init() to make sure that all the
original terminal settings are restored, as a curses-based alternative
spell checker (e.g. aspell) can change them

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@1866 35c25a1d-7b9e-4130-9fde-d3aeb78583b8