mgmt/mcumgr/lib: Copy mcumgr library under mgmt subsystem

The commit copies code of mcumgr, as a lib, under subsys/mgmt/mcumgr.
Only source files relevant for Zephyr have been copied, and changes
to source files have been limited only to path changes, where
required.
Samples have been left out as the only relevant sample, smp_svr,
is already within Zephyr source tree.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>