Added getnameinfo with only name lookup (#172)

Added necessary constants (DNS_TYPE_PTR, NI_NUMERICHOST etc.).
Implementation of getnameinfo is similar to getaddrinfo, with internal
functions:

* ResolveDnsReverse: performs rDNS query and parses the PTR record
* ResolveHostsReverse: reads /etc/hosts to map hostname to
  address

Earlier, the HOSTS.txt would only need to be sorted at loading time,
because the only kind of lookup was name -> address. Now since address
-> name lookups are also possible, so the HostsTxt struct, the sorting
method (and the related tests) was changed to reflect this.

libc/sock/sock.h

@@ -9,7 +9,13 @@ COSMOPOLITAN_C_START_
 
 #define INET_ADDRSTRLEN 22
 
-#define NI_DGRAM   0x10
+#define NI_NUMERICHOST 0x01
+#define NI_NUMERICSERV 0x02
+#define NI_NOFQDN      0x04
+#define NI_NAMEREQD    0x08
+#define NI_DGRAM       0x10
+
+#define NI_MAXHOST 0xff
 #define NI_MAXSERV 0x20
 
 #define htons(u16) bswap_16(u16)