Rearrange platform-specific handling in pmdaproc.sh.
Rearrange platform-specific handling in pmdaproc.sh.

Keep everything that does platform-specific stuff in one
place now, run early on, instead of all over the shop.

The code that handles determining whether the PCP::PMDA
Perl module is installed is also reworked to not fail in
the case where perl_opt is set, but user doesn't select
that option when Install'ing a PMDA.  This bit Mark with
the simple PMDA, which has both Perl and C variants.

ryandoyle/pcp_original,wuliming/pcp,andyvand/cygpcpfans,andyvand/cygpcpfans,aeg-aeg/pcpfans,mbaldessari/pcp,tjanez/pcp,andyvand/cygpcpfans,adfernandes/pcp,aeg-aeg/pcpfans,edwardt/pcp,prasincs/pcp,adfernandes/pcp,wuliming/pcp,prasincs/pcp,tjanez/pcp,ryandoyle/pcp_original,edwardt/pcp,tjanez/pcp,prasincs/pcp,mbaldessari/pcp,prasincs/pcp,tjanez/pcp,ryandoyle/pcp_original,prasincs/pcp,mbaldessari/pcp,ryandoyle/pcp_original,wuliming/pcp,andyvand/cygpcpfans,adfernandes/pcp,prasincs/pcp,andyvand/cygpcpfans,aeg-aeg/pcpfans,ryandoyle/pcp_original,adfernandes/pcp,adfernandes/pcp,mbaldessari/pcp,andyvand/cygpcpfans,wuliming/pcp,prasincs/pcp,prasincs/pcp,edwardt/pcp,tjanez/pcp,mbaldessari/pcp,aeg-aeg/pcpfans,andyvand/cygpcpfans,edwardt/pcp,edwardt/pcp,aeg-aeg/pcpfans,wuliming/pcp,mbaldessari/pcp,ryandoyle/pcp_original,tjanez/pcp,adfernandes/pcp,wuliming/pcp,edwardt/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,ryandoyle/pcp_original,aeg-aeg/pcpfans,ryandoyle/pcp_original,wuliming/pcp,edwardt/pcp,andyvand/cygpcpfans,adfernandes/pcp,edwardt/pcp,tjanez/pcp,tjanez/pcp,wuliming/pcp,adfernandes/pcp