illumos gate

When panic() is called from within ztest, the mdb ::status command isn't as useful as it could be since the global panicstr variable isn't updated. We should modify the function to make sure panicstr is set, so ::status can present the error message just like it does on a failed assertion.
For example, here's a core file suffering from this issue:
$ mdb core.0 
Loading modules: [ libumem.so.1 libc.so.1 libzpool.so.1 libcmdutils.so.1 libavl.so.1 libnvpair.so.1 ld.so.1 ]
> ::status
debugging core file of ztest (32-bit) from psurya-zloop.dcenter
file: /net/galaxy/data/jenkins/zfs-make/psurya/38679-trunk/proto/root_i386/usr/bin/i86/ztest
initial argv: /net/galaxy/data/jenkins/zfs-make/psurya/38679-trunk/proto/root_i386/usr/bin/i8
threading model: native threads
status: process terminated by SIGABRT (Abort), pid=12306 uid=1138 code=-1
> ::stack
libc.so.1`_lwp_kill+7(194, 6, f4830b48, fee2fea4, fef03000, 807bea0)
libc.so.1`raise+0x22(6, 0, f4830b88, fee0d003)
libc.so.1`abort+0xf3(807bea0, fed7c2f2, f4830bd4, fed58b74, fed95000, 10a)
libzpool.so.1`panic(fed6b7cc, f4830bd4, 0, fed6b7cc)
libzpool.so.1`panic+0x1f(fed6b7cc, 0, f4830bf8, fed04680, fffffffe, ffffffff)
libzpool.so.1`metaslab_trace_add+0x63(c000244, 873e700, 8959140, 1000, 0, 0)
libzpool.so.1`metaslab_group_alloc_holefill+0x12d(873e700, c000244, 1000, 0, 10a, 0)
libzpool.so.1`metaslab_group_alloc+0x8b(873e700, c000244, 1000, 0, 10a, 0)
libzpool.so.1`metaslab_alloc_dva+0x422(871a000, 8725900, 200, 0, 135cbc00, 0)
libzpool.so.1`metaslab_alloc+0x2b6(871a000, 8725900, 200, 0, 135cbc00, 3)
libzpool.so.1`zio_write_gang_block+0xd6(c000040, 1c, c000244, fed4c359)
libzpool.so.1`zio_dva_allocate+0x32f(c000040, 0, 0, fed4c3a6)
libzpool.so.1`zio_execute+0x1a2(c000040)
libcmdutils.so.1`taskq_thread+0x1c6(8726f10, 0, 0, fee8f3a4)
libc.so.1`_thrp_setup+0x8c(f8a10a40)
libc.so.1`_lwp_start(f8a10a40, 0, 0, 0, 0, 0)
> fed6b7cc/s
0xfed6b7cc:     too many entries in allocation list
> panicstr/s
libc.so.1`panicstr:
libc.so.1`panicstr:

Prior to this patch, libzpool's vpanic() function rolled it's own
mechanism for printing errors. Unfortunately, it wasn't properly setting
libc's panicstr variable, which meant mdb's '::status' command wouldn't
properly print the panic message.

This patch removes the custom error handling/printing code in libzpool's
vpanic() function, and uses assfail() instead. We couldn't use the
generic Abort() function since it isn't exported, and modifying the
interface of libc() is frowned upon.

assfail() was slightly modified to better handle this case, since it
would be awkward to print "assertion failed" when a panic() is
triggered. Thus, when a NULL filename is used, the message is slightly
tweaked to remove the reference to an assertion, and the filename and
line number isn't printed (it doesn't make sense to feed in the filename
and line number from vpanic() since it isn't a macro).