The OpenSSH keygen stores the key comment into a private key.
The Dropbear key format is simpler and can't do that.
But we can store/print it to a public key.

The option also improves compatibility with scripts developed for OpenSSH keygen.

Signed-off-by: Sergey Ponomarev <stokito@gmail.com>

The OpenSSH keygen stores the public part of a new key to a .pub file.
Make the DropBear behave same.

Signed-off-by: Sergey Ponomarev <stokito@gmail.com>

dprintf() was only introduced in posix 2008 so won't be supported
by older platforms. gnulib suggests:

https://www.gnu.org/software/gnulib/manual/html_node/dprintf.html
This function is missing on many non-glibc platforms: Mac OS X 10.5,
FreeBSD 6.0, NetBSD 5.0, OpenBSD 3.8, Minix 3.1.8, AIX 5.1, HP-UX 11,
IRIX 6.5, Solaris 11.3, Cygwin 1.5.x, mingw, MSVC 14.