net: refactor tftp library

- bugfix: Accept initial tftp server reply from a port different than
  the one used to establish the connection (typically 69) as mandated
  by RFC 1350. Previous implementation was not standard compliant.
- bugfix: close socket in case of error or timeout.
- bugfix: Reset retransmit counter after receipt of a good packet.
- bugfix: Use CONFIG_TFTP_LOG_LEVEL to set log level.
- api: upon successful receipt of the file set `client.user_buf_size`
  to the size of the file received.
- Restructure the code, comments.
- Limit usage of global variables.
- Limit usage of `goto`.

Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>