illumos gate

When running mdb's ::status command, certain assertion message appear to be truncated. For example, this message has its file and line number truncated:
$ mdb core.14 
Loading modules: [ libumem.so.1 libc.so.1 libzpool.so.1 libcmdutils.so.1 libavl.so.1 libnvpair.so.1 libtopo.so.1 ld.so.1 ]
> ::status
debugging core file of zdb (32-bit) from psurya-zloop-1.dcenter
file: /net/galaxy/data/jenkins/zfs-make/psurya/DLPX-34056/proto/root_i386/usr/sbin/i86/zdb
initial argv: /net/galaxy/data/jenkins/zfs-make/psurya/DLPX-34056/proto/root_i386/usr/sbin/i8
threading model: native threads
status: process terminated by SIGABRT (Abort)
panic message: assertion failed for thread 0xfec32a40, thread-id 1: svr->svr_vdev->vdev_removing, file ../../../uts/common/fs/zfs/vdev_removal

mdb currently uses a 128 character buffer to copy the 'panicstr' into
when displaying the string to the user; e.g. when using '::status'.
The functions collecting the panic message (e.g. assfail()) use a buffer
ranging from 400 bytes to 1000 bytes, so it makes sense for mdb to use a
buffer that is at least as big, if not bigger, than these so it can
print the full message. Thus, this patch increases the size of the
buffer mdb uses to print these messages from 128 bytes to 1024 bytes.