mpegts: Don't confuse slice allocator with regular one

Previously selector_bytes and private_data_bytes were sometimes allocated and
free using the normal allocator and sometimes using the slice allocator.
Additionally prefer g_strdup() to g_memdup() for strings.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=732789
mpegts: No need to check for NULL before calling g_free()

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=732789
glimagesink: Keep aspect ratio by default

The expected default behaviour for video sink is to maintain the
aspect ratio. Fix the default value to reflect this. The property
default was already TRUE, but the value was not initially TRUE.
watchdog: Add unit test

And improve some debug prints