Retain dynamic shared memory segments for postmaster lifetime

Currently there is no way user can keep the dsm
segments if he wants for postmaster lifetime, so I
have exposed a new API dsm_keep_segment()
to implement the same.

The specs and need for this API is already discussed
in thread:
http://www.postgresql.org/message-id/CA+TgmoaKoGuJQbEdGeYKYSXud9EAidqx77J2_HXzRgFo3Hr46A@mail.gmail.com

I had used dsm_demo (hacked it a bit) module used
during initial tests for dsm API's to verify the working on
Windows. So one idea could be that I can extend
that module to use this new API, so that it can be tested
by others as well or if you have any other better way, please
do let me know.

As the discussion about its specs and need is already
discussed in above mentioned thread, so I will upload
this patch to CF unless there is any Objection.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com