(PUP-9997) Remove call to Dir.chdir when managing symlinks

Symlinks can refer to their target using a relative or absolute path. If a
relative path is used, then it's assumed to be relative to the symlink's parent
directory. For example, /tmp/link refers to /tmp/target using a relative path:

    $ ls -l /tmp/link
    lrwxrwxrwx 1 josh 6 Jun 27 23:26 /tmp/link -> target

In commit 02f91fc, symlinks were merged into the file (aka pfile) type. The
`ensure` property was modified to validate whether the target existed or was a
directory[1] In order for relative symlinks to be validated, it was necessary
to call `Dir.chdir`.

Later the call to `Dir.chdir` was moved to `target.rb`[2], and then the
`FileTest.exists?(target)` check was deleted[3].

This commit removes the call to `Dir.chdir`. This means we no longer change our
cwd prior to dropping privileges and creating the symlink. This should be ok
because none of the code within the Dir.chdir block is sensitive to cwd. Also
when we reach this block of code, we're committed to creating the symlink given
an absolute `@resource[:path]`, so we don't need to be concerned about TOCTOU
race conditions.

[1] https://github.com/puppetlabs/puppet/blob/02f91fcd55aefe63a11a29c5608c0738867b8130/lib/puppet/type/pfile/ensure.rb#L76-L86
[2] https://github.com/puppetlabs/puppet/blob/2cd67ad843409a49747748a1278e5ef788dd97bd/lib/puppet/type/pfile/target.rb#L41-L45
[3] 662fcaf#diff-03f0464c0f42c4b979f6150ec2f7e8043ec964fe4d401cd0cd57a82f43a856ba

lib/puppet/type/file/target.rb

@@ -44,22 +44,20 @@ def mklink
 
  raise Puppet::Error, "Could not remove existing file" if Puppet::FileSystem.exist?(@resource[:path])
 
- Dir.chdir(File.dirname(@resource[:path])) do
- Puppet::Util::SUIDManager.asuser(@resource.asuser) do
- mode = @resource.should(:mode)
- if mode
- Puppet::Util.withumask(0o00) do
- Puppet::FileSystem.symlink(target, @resource[:path])
- end
- else
+ Puppet::Util::SUIDManager.asuser(@resource.asuser) do
+ mode = @resource.should(:mode)
+ if mode
+ Puppet::Util.withumask(0o00) do
  Puppet::FileSystem.symlink(target, @resource[:path])
  end
+ else
+ Puppet::FileSystem.symlink(target, @resource[:path])
  end
+ end
 
-  @resource.send(:property_fix)
+ @resource.send(:property_fix)
 
- :link_created
- end
+ :link_created
  end
 
  def insync?(currentvalue)