util: Add general low-level random bytes helper

Currently secret_init() open codes getting good quality random bytes from
the OS, either via getrandom(2) or reading /dev/random.  We're going to
add at least one more place that needs random data in future, so make a
general helper for getting random bytes.  While we're there, fix a number
of minor bugs:
 - getrandom() can theoretically return a "short read", so handle that case
 - getrandom() as well as read can return a transient EINTR
 - We would attempt to read data from /dev/random if we failed to open it
   (open() returns -1), but not if we opened it as fd 0 (unlikely, but ok)
 - More specific error reporting

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

util.h

@@ -263,6 +263,8 @@ static inline bool mod_between(unsigned x, unsigned i, unsigned j, unsigned m)
 /* FPRINTF() intentionally silences cert-err33-c clang-tidy warnings */
 #define FPRINTF(f, ...)	(void)fprintf(f, __VA_ARGS__)
 
+void raw_random(void *buf, size_t buflen);
+
 /*
  * Workarounds for https://github.com/llvm/llvm-project/issues/58992
  *