We want to add a command which doesn't need to run with privileges to
read the credential file. Initialization of the ua needs to read the
credential file. Factor out the code to initialize the ua so that we can
make that optional on a per-command basis.

Add new command `check-pbackup`, which makes suggestions, what jobs to
add to /etc/mxproxmox/backup-list.dat based on pbackup configuration.

The output consists of out-commented lines which can be added to
/etc/mxproxmox/backup-list.dat and uncommented one by one to phase in
new proxmox backup jobs.

Remove trailing whitespace from lines read from backup-list.

If we keep trailing whitespace, the following split will keep the
whitespace in $opt_limit. This is later checked with `if
defined($limit)` , which is true. Still, the empty string will evaluate
numerically to zero (with a warning). A limit zero is not what we want
here.

From `perldoc -f split`:

    When assigning to a list, if LIMIT is omitted, or zero, Perl supplies a
    LIMIT one larger than the number of variables in the list, to avoid
    unnecessary work

This has this surprising effect:

      DB<1> x split " ","a   "
    0  'a'
      DB<2> @a = split " ","a   "
      DB<3> x @s
      empty array
      DB<4> x @a
    0  'a'
      DB<5> ($a, $b) = split " ","a   "
      DB<6> x ($a, $b)
    0  'a'
    1  ''
      DB<7>