kernel/arch: streamline thread user options

The K_<thread option> flags/options avaialble to users were hidden in
the kernel private header files: move them to include/kernel.h to
publicize them.

Also, to avoid any future confusion, rename the k_thread.execution_flags
field to user_options.

Change-Id: I65a6fd5e9e78d4ccf783f3304b607a1e6956aeac
Signed-off-by: Benjamin Walsh <walsh.benj@gmail.com>