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/defs.h b/defs.h
index 4b9c29e..fc7e362 100644
--- a/defs.h
+++ b/defs.h
@@ -424,7 +424,7 @@
 
 struct xlat {
 	int val;
-	char *str;
+	const char *str;
 };
 
 extern const struct xlat open_mode_flags[];
@@ -488,9 +488,9 @@
 
 #define alloctcb(pid)	alloc_tcb((pid), 1)
 
-extern void set_sortby(char *);
+extern void set_sortby(const char *);
 extern void set_overhead(int);
-extern void qualify(char *);
+extern void qualify(const char *);
 extern int get_scno(struct tcb *);
 extern long known_scno(struct tcb *);
 extern long do_ptrace(int request, struct tcb *tcp, void *addr, void *data);
@@ -509,8 +509,8 @@
 extern void dumpiov(struct tcb *, int, long);
 extern void dumpstr(struct tcb *, long, int);
 extern void printstr(struct tcb *, long, int);
-extern void printnum(struct tcb *, long, char *);
-extern void printnum_int(struct tcb *, long, char *);
+extern void printnum(struct tcb *, long, const char *);
+extern void printnum_int(struct tcb *, long, const char *);
 extern void printpath(struct tcb *, long);
 extern void printpathn(struct tcb *, long, int);
 extern void printtv_bitness(struct tcb *, long, enum bitness_t, int);