cmake: support COMPONENTS param in Findpmem.cmake

cmake: support COMPONENTS param in Findpmem.cmake

add two components: pmem and pmemobj to this package. so we can find
them and link against them in a more intuitive way.

before this change the COMPONENTS parameter passed to

find_package(pmem ...)

is dropped on the floor and ignored.

after this change, it is checked and taken into consideration.

also, in this change, the exposed variables are renamed from

PMEM_* to pmem_*

to be consistent with the package name. it's encouraged to be consistent
with the package name when it comes to the INCLUDE_DIR and LIBRARIES
variable names.

Signed-off-by: Kefu Chai <kchai@redhat.com>