add execute_on_pre_bridge and execute_on_post_bridge vars to behave like the other family of execute_on and not break the existing pre bridge vars that are now depricated

Closes: FS-4241 (Thanks to Marek Wiewiorski for the patch)

Closes: FS-5224 (Thanks to Nathan Neulinger for the patch)

Closes: FS-5227 (Thanks to Nathan Neulinger for the patch)

Closes: FS-5218 (Thanks to Nathan Neulinger for the patch)

fs_encode: load mod_spandsp before loading mod_native_file so that the latter picks up all the codecs offered by the former

Apparently the Raspberry Pi blows up (out of memory) if you call
dpkg-source -z9 as it implicitly calls xz -9.  Thanks to Ken Rice for
running headlong into this.

Also note that while xz takes -0, dpkg-source won't take -z0, so we
check for this and make sure we call dpkg-source correctly.