slurmd - fix ABRT on SIGINT after reconfigure with MemSpecLimit set.

Use setenv() instead of setenvfs(), since setenvfs() memory allocation
is implemented with xmalloc() and fini_setproctitle() (which is called
on reconfigure) free's the memory with free(), leading to a:
"free(): invalid size" malloc_printerr error.

Continuation of dce83a23.

Bug 5095.