fix unlikely corner cases in getopt's message printing

like fputs (see commit 10a17dfbad), the
message printing code for getopt assumed that fwrite only returns 0 on
failure, but it can also happen on success if the total length to be
written is zero. programs with zero-length argv[0] were affected.

commit 500c6886c6 introduced this
problem in getopt by fixing the fwrite behavior to conform to the
requirements of ISO C. previously the wrong expectations of the getopt
code were met by the fwrite implementation.

src/misc/getopt.c

@@ -17,9 +17,9 @@ void __getopt_msg(const char *a, const char *b, const char *c, size_t l)
 	FILE *f = stderr;
 	b = __lctrans_cur(b);
 	flockfile(f);
-	fwrite(a, strlen(a), 1, f)
+	fputs(a, f)>=0
 	&& fwrite(b, strlen(b), 1, f)
-	&& fwrite(c, l, 1, f)
+	&& fwrite(c, 1, l, f)==l
 	&& putc('\n', f);
 	funlockfile(f);
 }