anetPeerToString(): make unix sockets distinguishable.
anetPeerToString(): make unix sockets distinguishable.

Following the CLIENT LIST output format, we prefix the unix socket
address with a "/" so that it is different than an IPv4/6 address.
This makes parsing simpler.

Related to #2010.

xuzhezhaozhao/redis_reading,ksarch-saas/redis,himoca/redis,ramonsnir/redis,spearhead-ea/redis,MSOpenTech/redis,maodeyi/redis_lua,MSOpenTech/redis,slfs007/mk-redis,darksideofthemoo/redis-histogram,ramonsnir/redis,xuzhezhaozhao/redis_reading,ksarch-saas/redis,himoca/redis,louisliangjun/redis,maodeyi/redis_lua,ttuna/msot-redis,spearhead-ea/redis,MSOpenTech/redis,HunanTV/redis,HunanTV/redis,HunanTV/redis,ramonsnir/redis,jxwr/redis,ttuna/msot-redis,louisliangjun/redis,xuzhezhaozhao/redis_reading,slfs007/mk-redis,ttuna/msot-redis,splitice/redis,spearhead-ea/redis,ttuna/msot-redis,splitice/redis,maodeyi/redis_lua,MSOpenTech/redis,jxwr/redis,splitice/redis,ksarch-saas/redis,darksideofthemoo/redis-histogram,MSOpenTech/redis,ttuna/msot-redis,ksarch-saas/redis,jxwr/redis,himoca/redis,HunanTV/redis,louisliangjun/redis,darksideofthemoo/redis-histogram,xuzhezhaozhao/redis_reading,louisliangjun/redis,splitice/redis,darksideofthemoo/redis-histogram,slfs007/mk-redis