Fix some gcc warnings and style suggestions

Building against gcc-10 produced warnings and style suggestions.
Fixed all the warnings, but only select style suggestions due to
readability and others appeared to be compiler confusion.

Tested: Compiled
    No functional impacts appear to have been made with these changes

Change-Id: I1adcce355b831ae8b178bfba9b89cc18fe4f3cf7
Signed-off-by: Matthew Barth <msbarth@us.ibm.com>
diff --git a/sdbusplus.hpp b/sdbusplus.hpp
index 9c3d08d..703895e 100644
--- a/sdbusplus.hpp
+++ b/sdbusplus.hpp
@@ -32,7 +32,7 @@
 class DBusError : public std::runtime_error
 {
   public:
-    DBusError(const char* msg) : std::runtime_error(msg)
+    explicit DBusError(const char* msg) : std::runtime_error(msg)
     {}
 };