Fix const-correctness issues uncovered by gcc -Wwrite-strings

* defs.h (struct xlat): Add const qualifier to the field of
type "char *".
(set_sortby, qualify, printnum, printnum_int): Add const qualifier to
arguments of type "char *".
* count.c (set_sortby): Add const qualifier to the argument and
automatic variable of type "char *".
* desc.c (decode_select): Add const qualifier to automatic variables of
type "char *".
* ioctlsort.c (struct ioctlent): Add const qualifier to fields of
type "char *".
(main):  Add const qualifier to argv.
* process.c (printargv): Add const qualifier to the argument and
automatic variable of type "char *".
(printargc) Add const qualifier to argument of type "char *".
* signal.c (sprintsigmask, parse_sigset_t): Add const qualifier to
arguments of type "char *".
* strace.c (progname): Add const qualifier.
(detach): Add const qualifier to automatic variable of type "char *".
* stream.c (struct strbuf): Add const qualifier to the field of
type "char *".
* syscall.c (struct qual_options): Add const qualifier to fields of
type "char *".
(qual_syscall, qual_fault, qual_desc, lookup_class): Add const qualifier
to arguments of type "char *".
(qual_signal): Add const qualifier to the argument of type "char *",
avoid modification of constant argument.
(qualify): Likewise.
* util.c (printflags): Add const qualifier to automatic variable of
type "char *".
(printnum, printnum_int): Add const qualifier to arguments of
type "char *".
diff --git a/util.c b/util.c
index 07cbfde..c28edf3 100644
--- a/util.c
+++ b/util.c
@@ -340,13 +340,10 @@
 }
 
 int
-printflags(xlat, flags, dflt)
-const struct xlat *xlat;
-int flags;
-const char *dflt;
+printflags(const struct xlat *xlat, int flags, const char *dflt)
 {
 	int n;
-	char *sep;
+	const char *sep;
 
 	if (flags == 0 && xlat->val == 0) {
 		tprintf("%s", xlat->str);
@@ -383,10 +380,7 @@
 }
 
 void
-printnum(tcp, addr, fmt)
-struct tcb *tcp;
-long addr;
-char *fmt;
+printnum(struct tcb *tcp, long addr, const char *fmt)
 {
 	long num;
 
@@ -404,10 +398,7 @@
 }
 
 void
-printnum_int(tcp, addr, fmt)
-struct tcb *tcp;
-long addr;
-char *fmt;
+printnum_int(struct tcb *tcp, long addr, const char *fmt)
 {
 	int num;