MEDIUM: Add external check

Add an external check which makes use of an external process to
check the status of a server.
diff --git a/include/common/defaults.h b/include/common/defaults.h
index c53db08..5508314 100644
--- a/include/common/defaults.h
+++ b/include/common/defaults.h
@@ -128,6 +128,7 @@
 #define DEF_AGENT_FALLTIME    1
 #define DEF_AGENT_RISETIME    1
 #define DEF_CHECK_REQ   "OPTIONS / HTTP/1.0\r\n"
+#define DEF_CHECK_PATH  ""
 #define DEF_SMTP_CHECK_REQ   "HELO localhost\r\n"
 #define DEF_LDAP_CHECK_REQ   "\x30\x0c\x02\x01\x01\x60\x07\x02\x01\x03\x04\x00\x80\x00"
 #define DEF_REDIS_CHECK_REQ  "*1\r\n$4\r\nPING\r\n"