dropbear server: support -T max auth tries

sysoptions.h: Add ability to override DROPBEAR_LISTEN_BACKLOG

Add support for '-T n' for a run-time specification for maximum number
of authentication attempts where 'n' is between 1 and compile time
option MAX_AUTH_TRIES.

A default number of tries can be specified at compile time using
'DEFAULT_AUTH_TRIES' which itself defaults to MAX_AUTH_TRIES for
backwards compatibility.

Signed-off-by: Kevin Darbyshire-Bryant <kevin@darbyshire-bryant.me.uk>

print key size with dropbearkey

This change allows adding DROPBEAR_LISTEN_BACKLOG to localoptions.h to
force the value.

Fix libtomcrypt/libtommath linking order

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;

Needed for curve25519-donna https://github.com/agl/curve25519-donna/issues/31