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/desc.c b/desc.c
index 9571e49..2b9f30a 100644
--- a/desc.c
+++ b/desc.c
@@ -490,7 +490,7 @@
 			       & -sizeof(long));
 	fd_set *fds;
 	static char outstr[1024];
-	char *sep;
+	const char *sep;
 	long arg;
 
 	if (entering(tcp)) {
@@ -529,7 +529,7 @@
 	else
 	{
 		unsigned int cumlen = 0;
-		char *sep = "";
+		const char *sep = "";
 
 		if (syserror(tcp))
 			return 0;