ebtables: fix segmentation fault due to uninitialized extension data

The ebtables code relies on the `-nostartfiles` linker argument to execute the
extension modules' `_init()` functions automatically which is not working
reliably across all supported targets and gcc versions.

Running an ebtables executable linked this way just crashes with a segmentation
fault at runtime on program startup, e.g. on ARM architectures.

In order to fix the issue ...
 - remove the use of the -nostartfiles linker flag
 - rename the init procedures to a generic name without implicit semantics
 - explicitely annotate those init procedures as constructors

The patch has been taken from the Alpine Linux distribution at
http://git.alpinelinux.org/cgit/aports/tree/main/ebtables/fix-extension-init.patch



Signed-off-by: Jo-Philipp Wich <jo@mein.io>