Add syscall uuidgen() for generating Univerally Unique Identifiers

(UUIDs). On ia64 UUIDs, aka GUIDs, are used by EFI and the firmware
among others. To create GUID Partition Tables (GPTs), we need to
be able to generate UUIDs.

sys/kern/syscalls.master

@@ -562,3 +562,4 @@
 390	STD	BSD	{ int kenv(int what, const char *name, char *value, \
 			    int len); }
 391	STD	BSD	{ int lchflags(const char *path, int flags); }
+392	STD	BSD	{ int uuidgen(struct uuid *store, int count); }