Also, strictly speaking, these numbers aren't DLT_*; they are LINKTYPE_* because
libpcap translates from internal OS-specific DLT_ numbering to the portable
LINKTYPE_ number space when writing files.
https://bugzilla.gnome.org/show_bug.cgi?id=738206
That is, output timestamps can then either be the absolute capture time,
or the relative capture time (w.r.t. to first output buffer), or the relative
capture time incremented by some offset.
Remove the weird (failing) code to figure out caps on the srcpad.
Add a caps property to decide what caps to put on the outgoing buffers.
Fix an event leak.
