core/parser: new internal msg flag - FL_UAC_AUTH

- to mark requests that had local uac authentication done (e.g., via
  uac_auth() from uac module)
- it is an indication that cseq should be increased
uac: mark request with FL_UAC_AUTH upon using uac_auth()
corex: functions for management of msg internal flags

- msg_iflag_set("flagname")
- msg_iflag_reset("flagname")
- msg_iflag_is_set("flagname")

- flagname can be: USE_UAC_FROM, USE_UAC_TO or UAC_AUTH
- this functions should not be used in configuration file for (re)setting
  the flags, those are done by various functions internally, however, in
  very particular cases they might be useful (e.g., changing From/To via
  textops functions)
dialog: add option to increment cseq upon local authentication to next hop

- feature has to be enabled via module parameter track_cseq_updates
- it does it only for downstream direction (requests from caller and
  callee, as it is the typical use case of calling via a provider, after
  authenticating the caller locally, provider asks for another 'trunk'
  authentication)
- diff of cseq value is stored in dialog var named 'cseq_diff',
  therefore be sure it is not overwritten from config
dialog: documented the track_cseq_updates module parameter
corex: documented functions used for management of sip msg internal flags