Enable -Wformat and -Wformat-security by default.

--- a/gcc/c-family/c.opt	2016-03-23 18:51:56.000000000 +0100
+++ b/gcc/c-family/c.opt	2016-04-28 23:45:54.063351272 +0200
@@ -459,7 +459,7 @@ C ObjC C++ ObjC++ Var(warn_format_nonlit
 Warn about format strings that are not literals.
 
 Wformat-security
-C ObjC C++ ObjC++ Var(warn_format_security) Warning LangEnabledBy(C ObjC C++ ObjC++,Wformat=, warn_format >= 2, 0)
+C ObjC C++ ObjC++ Var(warn_format_security) Init(1) Warning LangEnabledBy(C ObjC C++ ObjC++,Wformat=, warn_format >= 2, 0)
 Warn about possible security problems with format functions.
 
 Wformat-signedness
@@ -475,7 +475,7 @@ C ObjC C++ ObjC++ Var(warn_format_zero_l
 Warn about zero-length formats.
 
 Wformat=
-C ObjC C++ ObjC++ Joined RejectNegative UInteger Var(warn_format) Warning LangEnabledBy(C ObjC C++ ObjC++,Wall, 1, 0) IntegerRange(0, 2)
+C ObjC C++ ObjC++ Joined RejectNegative UInteger Var(warn_format) Init(1) Warning LangEnabledBy(C ObjC C++ ObjC++,Wall, 1, 0) IntegerRange(0, 2)
 Warn about printf/scanf/strftime/strfmon format string anomalies.
 
 Wignored-qualifiers