dropbear: server support option '-T' max auth tries

dropbear: server support option '-T' max auth tries

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>
(cherry picked from commit 9aaf3d3501642640c9ec74268e671171c8de6103)