Curl_setopt: handle arbitrary-length username and password

libcurl truncates usernames, passwords, and options set with
curl_easy_setopt to 255 (= MAX_CURL_PASSWORD_LENGTH - 1) characters.
This doesn't affect the return value from curl_easy_setopt(), so from
the caller's point of view, there is no sign anything strange has
happened, except that authentication fails.

For example:

  # Prepare a long (300-char) password.
  s=0123456789; s=$s$s$s$s$s$s$s$s$s$s; s=$s$s$s;
  # Start a server.
  nc -l -p 8888 | tee out & pid=$!
  # Tell curl to pass the password to the server.
  curl --user me:$s http://localhost:8888 & sleep 1; kill $pid
  # Extract the password.
  userpass=$(
	awk '/Authorization: Basic/ {print $3}' <out |
	tr -d '\r' |
	base64 -d
  )
  password=${userpass#me:}
  echo ${#password}

Expected result: 300
Actual result: 255

The fix is simple: allocate appropriately sized buffers on the heap
instead of trying to squeeze the provided values into fixed-size
on-stack buffers.

Bug: http://bugs.debian.org/719856
Reported-by: Colby Ranger