net: ppp: Fixes to peer LCP_OPTION_MRU handling

- Use the minimum of our and peer MRU as the MTU of the link.
  Allows for cases where our MRU is < 1500.

- Move all of the MRU handling under CONFIG_NET_L2_PPP_OPTION_MRU.
  So that we handle both sending the LCP_OPTION_MRU in Configure-Request
  and receiving it in Configure-Request.

- Remove CONFIG_NET_L2_PPP_OPTION_MAX_MRU.
  From RFC 1661, 6.1 Implementation note:
  ... The peer need not Configure-Nak to indicate that it will only send
  smaller packets, since the implementation will always require
  support for at least 1500 octets.

- Set ppp_my_options_parse_conf_ack so that ack's are parsed.

Signed-off-by: Markus Lassila <markus.lassila@nordicsemi.no>