Update URLs with https

Try to read from /dev/urandom multiple times, take input from extra sources,
and use /dev/random when generating private keys

if that wasn't what the client requested.

if public key authentication is disabled

- Fix build if ENABLE_SVR_REMOTETCPFWD is disabled but ENABLE_SVR_LOCALTCPFWD
  is enabled

specific IPv6 addresses.

From OpenWRT,
https://dev.openwrt.org/browser/trunk/package/dropbear/patches/300-ipv6_addr_port_split.patch

Got rid of ENABLE_AGENTFWD macro.

It makes it a bit more obvious why there's a test there since HAVE_FORK
is the normal case.

If we build out of tree and then run `make clean`, we hit an
infinite loop where libtommath tries to enter subdirs that don't
exist and run `make clean`.

the original_command into chansess struct since that makes more sense

  with a ChanType->closehandler()

IPV6_V6ONLY. From Gustavo Zacarias.