Replace use of unsafe string functions with snprintf if available.

This avoids warnings in OpenBSD that apparently can't be turned
off whenever you link strcpy, strcat, or sprintf.  When snprintf
isn't available, the use of the "unsafe" string functions has
always in fact been safe, since the lengths are all checked before
those functions are called.

We do not use strlcpy or strlcat, since they are not (yet) found on
all systems.  snprintf on the other hand is part of the C standard
library and is very common.

test/minigzip.c

@@ -463,8 +463,12 @@ void file_compress(file, mode)
         exit(1);
     }
 
+#if !defined(NO_snprintf) && !defined(NO_vsnprintf)
+    snprintf(outfile, sizeof(outfile), "%s%s", file, GZ_SUFFIX);
+#else
     strcpy(outfile, file);
     strcat(outfile, GZ_SUFFIX);
+#endif
 
     in = fopen(file, "rb");
     if (in == NULL) {
@@ -499,7 +503,11 @@ void file_uncompress(file)
         exit(1);
     }
 
+#if !defined(NO_snprintf) && !defined(NO_vsnprintf)
+    snprintf(buf, sizeof(buf), "%s", file);
+#else
     strcpy(buf, file);
+#endif
 
     if (len > SUFFIX_LEN && strcmp(file+len-SUFFIX_LEN, GZ_SUFFIX) == 0) {
         infile = file;
@@ -508,7 +516,11 @@ void file_uncompress(file)
     } else {
         outfile = file;
         infile = buf;
+#if !defined(NO_snprintf) && !defined(NO_vsnprintf)
+        snprintf(buf + len, sizeof(buf) - len, "%s", GZ_SUFFIX);
+#else
         strcat(infile, GZ_SUFFIX);
+#endif
     }
     in = gzopen(infile, "rb");
     if (in == NULL) {
@@ -546,7 +558,11 @@ int main(argc, argv)
     gzFile file;
     char *bname, outmode[20];
 
+#if !defined(NO_snprintf) && !defined(NO_vsnprintf)
+    snprintf(outmode, sizeof(outmode), "%s", "wb6 ");
+#else
     strcpy(outmode, "wb6 ");
+#endif
 
     prog = argv[0];
     bname = strrchr(argv[0], '/');