make the asynsend table size NPROCS-dependent.

this is a fix for e.g. the situation where lots of processes die
instantly, and PM has to send an asyn msg for each one to VFS, and
panics if there are too many. there are likely more situations in
which this table should be dependent on the no. of processes.

reported by pikpik on #minix3.

lib/libsys/asynsend.c

@@ -13,11 +13,12 @@
 #include <unistd.h>
 #include <minix/syslib.h>
 #include <minix/sysutil.h>
+#include <minix/sys_config.h>
 
 #include <limits.h>
 #include <errno.h>
 
-#define ASYN_NR	100
+#define ASYN_NR	(2*_NR_PROCS)
 PRIVATE asynmsg_t msgtable[ASYN_NR];
 PRIVATE int first_slot= 0, next_slot= 0;