Add UserAgentTokens config directive (Issue #5555)

Add UserAgentTokens config directive (Issue #5555)

cups/cups-private.h:
- Add enum and value for UserAgentTokens directive.

cups/usersys.c:
- _cups_finalize_conf(): Copy UserAgentTokens value to globals.
- _cups_init_conf(): Set default UserAgentTokens value and grab prefs.
- _cups_read_conf(): Look for UserAgentTokens value.
- _cups_set_uatokens(): Set UserAgentTokens value.
- cupsSetUserAgent(): Support UserAgentTokens values.