In CANNOT_DUMP builds, don't prepare for unexec.

In CANNOT_DUMP builds, don't prepare for unexec.

Having a command-line argument of "dump" or "bootstrap" would trigger
behavior like not installing signal handlers.  In CANNOT_DUMP modes,
we should get signal handlers installed regardless of whatever funny
file names we decide to edit.

src/emacs.c (main) [CANNOT_DUMP]: Don't enable the "dumping"
alterations to initialization that prepares the process for unexec.