clang compiler warnings: Fix invalid enum conversion

This patch fixes some invalid enum conversion warnings caught by clang. In
particular:
* chan_sip: Several functions mixed usage of the st_refresher_param
  enum and st_refresher enum. This patch corrects the functions to use the
  right enum.
* chan_pjsip: Fixed mixed usage of ast_sip_session_t38state and ast_t38_state.
* strings: Fixed incorrect usage of AO2 flags with strings container.
* res_stasis: Change a return enumeration to stasis_app_user_event_res.

Review: https://reviewboard.asterisk.org/r/4535

ASTERISK-24917
Reported by: dkdegroot
patches:
  rb4535.patch submitted by dkdegroot (License 6600)
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Merged revisions 433746 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 433747 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@433748 65c4cc65-6c06-0410-ace0-fbb531ad65f3