Add uuid wrapper API call ast_uuid_generate_str().

Add uuid wrapper API call ast_uuid_generate_str().

* Updated test_uuid.c to test the new API call.

* Made system use the new API call to eliminate "10's of lines" where
used.

* Fixed untested ast_strdup() return in stasis_subscribe() by eliminating
the need for it.  struct stasis_subscription now contains the uniqueid[]
string.

* Fixed some issues in exchangecal_write_event():
  Create uid with enough space for a UUID string to avoid a realloc.
  Fix off by one error if the calendar event provided a UUID string.
  There is no need to check for NULL before calling ast_free().

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@384302 65c4cc65-6c06-0410-ace0-fbb531ad65f3