kernel: some boottime sanitychecks

. Check if we have the right number of boot modules
	. Check if the ELF parsing of VM actually succeeded

Both these are root causes of less-than-obvious other
errors/asserts a little further down the line; uncovered
while experimenting with booting by iPXE, specifically
(a) iPXE having a 8-multiboot-modules limit and
(b) trying to boot a gzipped VM.

include/minix/com.h

@@ -84,6 +84,7 @@
                                            computation in macros.*/
 #define INIT_PROC_NR ((endpoint_t) LAST_SPECIAL_PROC_NR)  /* init
                                                         -- goes multiuser */
+#define NR_BOOT_MODULES (INIT_PROC_NR+1)
 
 /* Root system process and root user process. */
 #define ROOT_SYS_PROC_NR  RS_PROC_NR