Switch to use Requires.private for devmapper.pc and lvm2app.pc

Switch to use Requires.private for devmapper.pc and lvm2app.pc

Use Requires.private: instead of Libs.private:
Use UDEV_PC and SELINUX_PC for Require.private:

It looks like usage of Requires.private is prefered from Libs.private.
However pkg-config documentation is really poor here. But here is
short outcome:

There is a difference in Libs.private: and Requires.private: where
we specify libselinux instead of  -lselinux -lsepol.

We leave resolving of query like 'pkg-config --libs --static devmapper'
on taking proper selinux and udev libs to their .pc files instead of
hardcoding them into our .pc file which is might give incorrect answer.
- i.e. dependency of libselinux package might change and we may return
wrong list of linked libraries.

http://bugs.freedesktop.org/show_bug.cgi?id=4738
http://err.no/personal/blog/tech/2008-03-25-18-07_pkg-config,_sonames_and_Requires.private