core cleanup: move sipe_get_useragent() to sip-transport
Moved purple_core_get_version() to backend.
Replaced static variable default_ua with a variable in sipe_core_private. This
fixes the bug that the same useragent value was used for all accounts although
they might be configured by the user with different useragent settings.
This also means one less global variable, ie. one step closer to remove the
requirement for threaded backends to maintain a global lock instead of an
connection local lock.