ps: Increase command selection field to 64

The library now presents command names up to 64 characters, in line with
the kernel changes.  ps command name selection (the -C option) now also
is 64 characters long.

References:
 commit 2cfdbbe897

ps/common.h

@@ -189,7 +189,7 @@ typedef union sel_union {
   uid_t uid;
   gid_t gid;
   dev_t tty;
-  char  cmd[16];  /* this is _not_ \0 terminated */
+  char  cmd[64];  /* this is _not_ \0 terminated */
 } sel_union;
 
 typedef struct selection_node {