Catch attempts to fetch information about non-activated pcap_t's.

The link-layer header type, and list of supported link-layer header
types, aren't set until the pcap_t is activated; return an error for
attempts to fetch them for a non-activated handle.
Just return PCAP_ERROR_NOT_ACTIVATED for some calls.

Return PCAP_ERROR_NOT_ACTIVATED, not PCAP_ERROR (-1), for calls to
pcap_datalink(), pcap_datalink_ext(), and pcap_list_datalinks() if
called on a non-activated pcap_t.  Pre-pcap_create() code will never see
PCAP_ERROR_NOT_ACTIVATED, and code using pcap_create() will either not
see it (if it's correct) or is already getting bogus results (if it's
not correct).

Document that while we're at it.
Add more "not activated" checks.

Add a missing semicolon from the previous checkin, and change some
return values to the standard style (enclosing the return value in
parentheses), while we're at it.