elfcopy: include extension but replace . when converting from binary

The change in r291958 was not consistent with GNU objcopy. The start,
end and size symbols created for ELF objects converted from binary need
to include the full filename including the extension, but with the
periods replaced with underscores.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D4474