Consolidate and harden SlurmctldHost and ControlMachine config parsing.

Collapse into a single function so we can appropriately warn
if a mix of options are in use.

This also avoids a confusing-looking xmalloc with the count padded by two,
which was being used to build out space for ControlMachine if SlurmctldHost
was not defined. This would have also masked off a series of off-by-one
errors, and has lead to attempts to connect to 0.0.0.0 instead of a segfault.
(Some code was intentionally using this over-provisioning as a way to
treat this as a NULL-terminated list, but this was then technically
incorrect in cases where the old-style BackupController was set since the
NULL would happen at the third position in the array, which is an invalid
memory access.)