Starting back to front:

Conclusion: I think ImageMagick should be compiled with
HasSharedMemory unset (at least from my i386/current point of view).

Why bother mentioning it:
Either patch-aa of the package source has

-XCOMM #define DoSharedLib  HasSharedLibraries
+#define DoSharedLib  HasSharedLibraries

and maybe shouldn't have (though I didn't use the package system to
build it)

or something needs editing in my kernel config

options         SYSVMSG         # System V-like message queues
options         SYSVSEM         # System V-like semaphores
options         SYSVSHM         # System V-like memory sharing
#options        SHMMAXPGS=1024  # 1024 pages is the default

or something needs to be added to sysvshm

 16929 display  CALL  #231 (unimplemented shmget)(0,0x4b000,0x3ff)
 16929 display  RET   #231 (unimplemented shmget) 131075/0x20003
 16929 display  CALL  #228 (unimplemented shmat)(0x20003,0,0)
 16929 display  RET   #228 (unimplemented shmat) 1141227520/0x4405c000
...
 16929 display  CALL  #229 (unimplemented shmctl)(0xe0002,0,0)
 16929 display  RET   #229 (unimplemented shmctl) 0
 16929 display  CALL  exit(0xb)


Observed brokeness:
When using ImageMagick 4.0.6 compiled with HasSharedMemory defined
under NetBSD-current/i386, if I "display" an image, then file open
another, "display" successfully loads the image, classifies its
colours and dither the image. Then it promptly exits. In the source,
the various parts of magick/X.c containing shm* calls are protected
using #ifdef HasSharedMemory. All OK without HasSharedMemory.

Cheers,

Patrick