pre-PPP cross protocol bits

Factored these non-PPP-specific bits out from the ppp_rebased branch.

Most of these are functions that are used in Juniper/Pulse code which are also useful for PPP.

• make buf_append_{be16,be32,le16} global
• auth-juniper.c simplifications (including ignoring submit_button if NULL)
  • factor out general HTML-form-scraping functions from auth-juniper.c to auth-html.c
  • split htmlnode_next and htmlnode_dive
• factor out internal_get_url function
• factor out internal_split_cookies function
• universal MTU calculator for use with GPST and PPP
• add openconnect__strchrnul function to compat.c (useful for parsing)
• fix HTML parsing bug for <select>/<option>
  • not apparent due to structure of Juniper forms, but noticed in testing PPP protocols