dpmi: shorten shmname for ldt_alias

On Mac, PSHMNAMLEN is only 31, so change the shmname from something
like /dosemu_dpmishm_16495_ldt_alias_16495FPd to
/dosemu_ldt_alias_16495FPd which is still sufficiently unique.

src/dosext/dpmi/memory.c

@@ -599,7 +599,7 @@ dpmi_pm_block *DPMI_mallocShared(dpmi_pm_block_root *root,
         return NULL;
     }
 
-    asprintf(&shmname, "/dosemu_dpmishm_%d_%s", getpid(), name);
+    asprintf(&shmname, "/dosemu_%s", name);
     if (init) {
         oflags |= O_CREAT;
         if (flags & SHM_EXCL)