Don't try to create tinc.conf when using set or add commands.

It is almost certainly an error. If one really wants to create a new tinc.conf
file, one should use the init command.
Modernize the configure script a bit.
Use conditional compilation for device.c.

This requires the automake option "subdir-objects" to be enabled, and it
becomes more critical to specify the exact path to local header files.
Use conditional compilation for cryptographic functions.

This gets rid of the rest of the symbolic links. However, as a consequence, the
crypto header files have now moved to src/, and can no longer contain
library-specific declarations. Therefore, cipher_t, digest_t, ecdh_t, ecdsa_t
and rsa_t are now all opaque types, and only pointers to those types can be
used.