core/parser: Fix extract_sendrecv_mode to always return mode if valid

- All four sendrecv directions return 0
- is_on_hold flag set for inactive and sendonly
sdpops/documentation: Add id's to function sections
tm: Export additional functions from tm modules

- t_load_contacts and t_next_contacts in tm
- t_on_branch_failure in tmx
app_lua: Export additional tm functions

- sr.tm.t_load_contacts
- sr.tm.t_next_contatcs
- sr.tm.t_on_branch_failure
registrar: Prevent incorrect application of path when forwarding a register

- Reset the internal path pointer after a save() to prevent it being added
  as a Route: header, e.g. when running t_replicate()