broker/pmiutil: use larger kvsname buffer

Problem: openpmix's libpmi.so PMI_KVS_Get_my_name() fails with
PMI_ERR_INVALID_LENGTH if the kvsname buffer size is less than
its internal PMIX_MAX_NSLEN (255) value, regardless of the size
of the returned string.

Work around this by raising the broker's kvsname buffer size
from 64 to 1024 bytes.

Fixes #2746

src/broker/pmiutil.h

@@ -14,7 +14,7 @@
 struct pmi_params {
     int rank;
     int size;
-    char kvsname[64];
+    char kvsname[1024];
 };
 
 struct pmi_handle;