virrandom: make virRandomInitialize an automatic one-shot

All callers used the same initialization seed (well, the new
viratomictest forgot to look at getpid()); so we might as well
make this value automatic.  And while it may feel like we are
giving up functionality, I documented how to get it back in the
unlikely case that you actually need to debug with a fixed
pseudo-random sequence.  I left that crippled by default, so
that a stray environment variable doesn't cause a lack of
randomness to become a security issue.

* src/util/virrandom.c (virRandomInitialize): Rename...
(virRandomOnceInit): ...and make static, with one-shot call.
Document how to do fixed-seed debugging.
* src/util/virrandom.h (virRandomInitialize): Drop prototype.
* src/libvirt_private.syms (virrandom.h): Don't export it.
* src/libvirt.c (virInitialize): Adjust caller.
* src/lxc/lxc_controller.c (main): Likewise.
* src/security/virt-aa-helper.c (main): Likewise.
* src/util/iohelper.c (main): Likewise.
* tests/seclabeltest.c (main): Likewise.
* tests/testutils.c (virtTestMain): Likewise.
* tests/viratomictest.c (mymain): Likewise.

tests/viratomictest.c

@@ -165,8 +165,6 @@ mymain(void)
 {
     int ret = 0;
 
-    if (virRandomInitialize(time(NULL)) < 0)
-        return -1;
     if (virThreadInitialize() < 0)
         return -1;