Some tidying in installman.

Some tidying in installman.

Avoid the unnecessary lexicals $xnew and $xold.
Use lexicals for file handles, and check the return of close.
Why import mkpath() from File::Path and then comment its provenance at the
point of use?
Remove an exit(0) which is only 20 lines from the end of the script.
Note why we can't re-use a file handle open on the Pod file we're currently
processing.