wrapper: use a CSPRNG to generate random file names

The current way we generate random file names is by taking the seconds
and microseconds, plus the PID, and mixing them together, then encoding
them.  If this fails, we increment the value by 7777, and try again up
to TMP_MAX times.

Unfortunately, this is not the best idea from a security perspective.
If we're writing into TMPDIR, an attacker can guess these values easily
and prevent us from creating any temporary files at all by creating them
all first.  Even though we set TMP_MAX to 16384, this may be achievable
in some contexts, even if unlikely to occur in practice.

Fortunately, we can simply solve this by using the system
cryptographically secure pseudorandom number generator (CSPRNG) to
generate a random 64-bit value, and use that as before.  Note that there
is still a small bias here, but because a six-character sequence chosen
out of 62 characters provides about 36 bits of entropy, the bias here is
less than 2^-28, which is acceptable, especially considering we'll retry
several times.

Note that the use of a CSPRNG in generating temporary file names is also
used in many libcs.  glibc recently changed from an approach similar to
ours to using a CSPRNG, and FreeBSD and OpenBSD also use a CSPRNG in
this case.  Even if the likelihood of an attack is low, we should still
be at least as responsible in creating temporary files as libc is.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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