Use a proper PRNG for GTK askpass.

Coverity complained that it was wrong to use rand() in a security
context, and although in this case it's _very_ marginal, I can't
actually disagree that the choice of which light to light up to avoid
giving information about passphrase length is a security context.

So, no more rand(); instead we instantiate a shiny Fortuna PRNG
instance, seed it in more or less the usual way, and use that as an
overkill-level method of choosing which light to light up next.

(Acknowledging that this is a slightly unusual application and less
critical than most, I don't actually put the passphrase characters
themselves into the PRNG, and I don't use a random-seed file.)

Recipe

@@ -376,7 +376,7 @@ pageant  : [X] uxpgnt uxagentc aqsync pageant sshrsa sshpubk sshdes ARITH
 	 + sshmd5 version tree234 misc sshaes sshsha sshdss sshsh256 sshsh512
 	 + sshecc CONF uxsignal nocproxy nogss be_none x11fwd ux_x11 uxcons
          + gtkask gtkmisc nullplug logging UXMISC uxagentsock utils memory
-	 + sshauxcrypt sshhmac
+	 + sshauxcrypt sshhmac sshprng uxnoise
 
 ptermapp : [XT] GTKTERM uxmisc misc ldisc settings uxpty uxsel BE_NONE uxstore
          + uxsignal CHARSET uxpterm version time xpmpterm xpmptcfg