mkrockspecs: reset LUA_INIT and LUA_INIT_5_2 before re-exec.

* src/merge-sections.in, src/mkrockspecs.in: Add 'Bourne compat'
boilerplate, and ensure LUA_INIT and LUA_INIT_5_2 are reset in
the environment before re-execing with $LUA.
* build-aux/merge-sections: Regenerate.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
bootstrap: sync with upstream.

* build-aux/bootstrap.in: Sync with upstream bootstrap.
* bootstrap: Regenerate.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>