(#21241) Add a priority setting

This commit add a `:priority` type of setting that maps generic priority
labels to operating system specific priorities, e.g. normal priority is
0 on posix, but 32 on Windows.

If the value is specified as an integer, then it is passed as is, so
that specific priority levels can be specified.

This commit also adds the `priority` setting to the main section of
puppet's configuration.
(#21241) Set process priority if one is specified

This commit adds a Puppet::Util::Limits module for specifying process
priority, and mixes the module into Puppet::Util::CommandLine. Since
the CommandLine is the application entry point for puppet it seemed
sensible to have it set the process priority there, and to have it done
as early as possible.

Another option would have been to set the priority in the base
Application, after the application-specific settings have been
initialized. But this would mean you couldn't set the process priority
when puppet executes an ExternalSubcommand-based application.

As it stands now, it's not possible to have different priorities for
different sections, e.g. agent vs master, in puppet's configuration
file.
Merge pull request #1922 from joshcooper/ticket/master/21241-priority

(#21241) Allow process priority to be specified
(#21241) Use correct constant for low priority

Needs to be BELOW_NORMAL_PRIORITY_CLASS.
(#23074) Add reinventory command to puppet cert

This adds a reinventory command to the puppet cert application.

Further work on this is to refactor the use of the #apply method
found in Puppet::SSL::CertificateAuthority as it delegates to
a subclass called Interface to do the work. This is all an application
concern and it is confusing with the back and forth between the classes
"the cert application", the CA and its Interface.
(#23074) Add text to warn that master should be inactive for reinventory.
(#23074) Cleanup logic

This makes a couple of the methods in the CertificateAuthority and
related Interface classes easier to read / understand.
(#23074) Remove header in inventory.txt when regenerating

The regenerate inventory wrote a header that was not there in the
original file.
(#23074) Make cert inventory not rebuild inventory.txt when adding

There is a race condition that requires protection if the inventory
performs rebuild of a missing inventory.txt file when adding a cert.
Without the protection the inventory may end up with double entries or
missing entries, and with the protection the processing of cert requests
become serialized over all processors creating a performance problem.

This change simply appends to the inventory.txt - if the file does not
exists, it is atomically created.

A users that wants to recreate the inventory should do so with the
command `puppet cert reinventory` which may be executed on an
inactive system.
(#23074) Refactor cert application's use of cert authority's interface

The code was difficult to understand as the cert application called
the apply function on the CertAuthority, which in turned created an
instance of the Interface class, which in turn called back into the
ca. As this construct was only used by the cert application, the ca's
apply method was moved to the cert application.

It is now more clear that the application interacts with the ca via
an instance of its Interface class.