Add a local implementation of hstrerror for the system which do not have it

The function converts the value of h_errno (last error of name
resolver library, see netdb.h).
One of systems which supposedly do not have the function is SunOS.
POSIX does not mandate its presence.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/hstrerror.c

@@ -0,0 +1,21 @@
+#include <string.h>
+#include <stdio.h>
+#include <netdb.h>
+
+const char *githstrerror(int err)
+{
+	static char buffer[48];
+	switch (err)
+	{
+	case HOST_NOT_FOUND:
+		return "Authoritative answer: host not found";
+	case NO_DATA:
+		return "Valid name, no data record of requested type";
+	case NO_RECOVERY:
+		return "Non recoverable errors, FORMERR, REFUSED, NOTIMP";
+	case TRY_AGAIN:
+		return "Non-authoritative \"host not found\", or SERVERFAIL";
+	}
+	sprintf(buffer, "Name resolution error %d", err);
+	return buffer;
+}