unexec function prototype

[Dan Nicolaescu]

We currently get a warning in emacs.c because "unexec" does not have a
prototype.

More, the last two arguments are always zero, and the third is never
used in any implementation in unex*.c.

The bss_start argument is only used in unexaix.c and unexcoff.c, and
the code could be simplified because it's always passed as zero.

So it seems that "unexec" should only have a two arguments.

If there's no objection, I plan to change the code to do that.