ftmod_libpri: Initial support for BRI and various cleanups + double locking fix

ftmod_libpri: Initial support for BRI and various cleanups + double locking fix

Fixes:
- Fix double locking in check_state(): remove ftdm_mutex_(un)lock calls.

New:
- Add lpwrap_init_bri() to initialize libpri in BRI mode
- Use trunk type to determine PRI/BRI and PTP/PTMP mode (no additional configuration required).
- Improve libpri detection code in configure.ac, check whether libpri is usable
  and if BRI support is available (needs more work)

Cleanups:
- Whitespace and indentation cleanups
- Rename str2* functions to parse_*
- Simplify libpri_configure_span() option parsing and bail out on error
- Add "ftdm libpri help" command that displays usage information
- Coding style cleanups and other (minor) changes

Known issues:
- BRI PTMP TE Q.921 link does not go beyond TEI ASSIGNED state,
  SABME requests are ignored by NT (EuroISDN DTAG, DAHDI-2.3.0.1, vzaphfc)

Signed-off-by: Stefan Knoblich <s.knoblich@axsentis.de>