more lintings

Fix configure options using AC_ARG_ENABLE

fix indentation

Add .gitignore file

termcodes: make VEOL2, VWERASE, VLNEXT, ECHOCTL, and ECHOKE optional

Change label name _ERR to LBL_ERR

Use DROPBEAR_PATH_DEVNULL instead of undefined _PATH_DEVNULL

in function tcp_acceptor()
  struct sockaddr_storage addr;
  char* addr = NULL;

more linting

remove K&R old style code

in order to remove K&R code in atomicio.c

now, vwrite comes from atomicio.h

in order to remove K&R code

Configure options to disable some features, which are checked using
AC_ARG_ENABLE can be invoked with --enable-<option> as well.
The current configure script always generates a #define for such options
even though the --enable-<option> case should not generate a #define.
Fix this by properly checking if the feature should be enabled, in which
case no #define is being generated.

Signed-off-by: Stefan Hauser <stefan@shauser.net>