Change field name to power in slurm_opt_t, and update all locations.

Remove casts through (char *) now that power_flags_id() takes a
(const char *) as input.

The salloc command did not reset on each pass - or ever - so follow
the established behavior of sbatch/srun and reset for each component.

Note that invalid env vars will result in exit(-1); instead
of just an error message.

Drop it from sbatch since it just printed an error if you tried to use it;
getopt_long() will happily tell you that option isn't available itself.

Change slurm_opt_t field to 'exclude' to match command line.

Node that the _valid_node_list() call is being moved into _opt_verify()
where it probably should have been before. Since _valid_node_list()
depends on several other settings, processing it when --exclude is
processed may have resulted in inconsistent behavior if the order of the
various arguments was changed around.

Note that srun reset this only once, while salloc/sbatch reset
each pass. Go with salloc/sbatch behavior - I do not think
carrying the distribution over between components makes sense
for most allocations at this time.

Note this overlaps with the --exclusive option, but inverts
the default behavior if no arguments are given.

Note that srun was previously using NO_VAL as unset which is technically
an acceptable value; change it to -1 to match salloc/sbatch.

Note changes needed for both the environment variable handling, as well
as for PBS option parsing; have them call in to slurm_process_option().

Simplify error handling, and explicitly test for NO_VAL as a return from
time_str2mins().

There are two equivalent long-form options. Add a new LONG_OPT_CLUSTER
to the enum to distinguish which form is used. (Needed for tracing at
a later point in time.)

Neither salloc/sbatch reset this. I believe reseting each pass,
as is done by srun, is the correct behavior, otherwise each separate
component will request additional licenses which is unintuitive.

For reset, salloc always reset this where sbatch/srun would not.
Choosing to go with sbatch/srun's behavior.