mmu: Fix mmap() to fail if address space is exhausted

Throw an exception from find_hole() if address space is exhausted. This
is needed for supporting MAP_32BIT properly.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Remove dead code umem.c

This removes the opensolaris umem.c defining memory allocation functions
which we do not use.

We should also remove umem.h from the same directory, but the messy
code makes this very difficult... It turns out that we have two
zfs_context.h versions, and one of them includes umem.h. If I try
to remove this version of zfs_context.h, compilation breaks. It
will take more effort to clean this mess up :(

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
app: fix osv::application never getting destroyed

Fixes #434.

The 'app' variable never went out of scope causing shared_ptr to
remain live.

Tested-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>