[core] defer li_rand_init() until first use

[core] defer li_rand_init() until first use

defer li_rand_init() until first use of li_rand_pseudo_bytes()

li_rand_init() is now deferred until first use so that installations
that do not use modules which use these routines do need to potentially
block at startup.  Current use by core lighttpd modules is in mod_auth
HTTP Digest auth and in mod_usertrack.  Deferring collection of random
data until first use may allow sufficient entropy to be collected by
kernel before first use, helping reduce or avoid situations in
low-entropy-generating embedded devices which might otherwise block
lighttpd for minutes at device startup.  Further discussion in
https://redmine.lighttpd.net/boards/2/topics/6981