drivers: nsos: handle multiple blocking APIs on single socket

So far only a single blocking API could be handled simultaneously, due to
epoll_ctl(..., EPOLL_CTL_ADD, ...) returning -EEXIST when same file
descriptor was added twice.

Follow 'man epoll' advice about using dup() syscall to create a duplicate
file descriptor, which can be used with different events masks. Use such
duplicate for each blocking API except ioctl() (for handling Zephyr poll()
syscall).

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>