Fixed account's route set update when modifying account

[sauwming]

There's an issue with account modification via pjsua_acc_modify() when updating account's route_set.

Account's route set is a list that consists of: global proxies + account proxies + service route learnt from update_service_route().
But when modifying account, the learnt service route is never taken into account, causing incorrect list size calculation, and hence incorrect element removal.

The fixes in this PR:

• Learnt route headers is only removed upon registrar change. Account proxy is only modified upon change.
• Remove global outbound proxy modification.
  The current implementation seems to be buggy and not working as intended for the following reasons:
  • there's no API to modify the global proxy,
  • even if app modifies pjsua_var.ua_cfg.outbound_proxy internally (such as by including pjsua_internal.h), the route set is built from the list pjsua_var.outbound_proxy, which actually has different content and is never updated.
• Add new list APIs pj_list_insert_list_before/after().

Thank you to @andreas-wehrmann for the report and fix recommendation.

[nanangizz]

This change seems to drop the existing memory optimization, e.g: currently (shallow) cloning global proxy will only occur if global proxy is changed & (deep) cloning account proxy only occur if account proxy is changed, now both clonings will always occur when whichever proxy setting is changed or reg URI is changed. If some app for some reason modifies registrar URI regularly, this may cause unnecessary/avoidable "memory leak" (as account pool is never reset?).

If the memory optimization can be maintained, it will be ideal, but the new behavior may also be acceptable considering the memory leak scenario seems to be a rare scenario.

[sauwming]

Changes in the latest commit:

• Learnt route headers is only removed upon registrar change. Account proxy is only modified upon change.
• Remove global outbound proxy modification.
  The current implementation seems to be buggy and not working as intended for the following reasons:
  • there's no API to modify the global proxy,
  • even if app modifies pjsua_var.ua_cfg.outbound_proxy internally (such as by including pjsua_internal.h), the route set is built from the list pjsua_var.outbound_proxy, which actually has different content and is never updated.
• Add new list APIs pj_list_insert_list_before/after(). The current APIs pj_list_insert_nodes_before/after() seem to be useless since our list typically has an empty element as its head.