Avoid to concatenate extra leading slash

TODO: All usage of set_dir_bin and set_dir_src should be okay except the
TODO: following one in util.c:
TODO:
TODO:    538         /* If root is not md(4) and we have set dir, skip this step. */
TODO:    539         mib[0] = CTL_KERN;
TODO:    540         mib[1] = KERN_ROOT_DEVICE;
TODO:    541         varlen = sizeof(rootdev);
TODO:    542         (void)sysctl(mib, 2, rootdev, &varlen, NULL, 0);
TODO:    543         if (stat(set_dir_bin, &sb) == 0 && S_ISDIR(sb.st_mode) &&
TODO:    544             strncmp("md", rootdev, 2) != 0) {
TODO:    545                 strlcpy(ext_dir_bin, set_dir_bin, sizeof ext_dir_bin);
TODO:    546                 strlcpy(ext_dir_src, set_dir_src, sizeof ext_dir_src);
TODO:    547                 return SET_OK;
TODO:    548         }
TODO:
TODO: ..and the following in net.c:
TODO:
TODO:   1003         /* If root is on NFS and we have sets, skip this step. */
TODO:   1004         if (statvfs(set_dir_bin, &sb) == 0 &&
TODO:   1005             strcmp(sb.f_fstypename, "nfs") == 0) {
TODO:   1006                 strlcpy(ext_dir_bin, set_dir_bin, sizeof ext_dir_bin);
TODO:   1007                 strlcpy(ext_dir_src, set_dir_src, sizeof ext_dir_src);
TODO:   1008                 return SET_OK;
TODO:   1009         }
TODO:

Index: main.c
===================================================================
RCS file: /cvsroot/src/usr.sbin/sysinst/main.c,v
retrieving revision 1.11
diff -u -p -r1.11 main.c
--- main.c	6 Oct 2018 18:45:37 -0000	1.11
+++ main.c	20 Oct 2018 11:54:10 -0000
@@ -94,8 +94,8 @@ static const struct f_arg fflagopts[] = 
 	{"ftp host", SYSINST_FTP_HOST, ftp.xfer_host[XFER_FTP], sizeof ftp.xfer_host[XFER_FTP]},
 	{"http host", SYSINST_HTTP_HOST, ftp.xfer_host[XFER_HTTP], sizeof ftp.xfer_host[XFER_HTTP]},
 	{"ftp dir", SYSINST_FTP_DIR, ftp.dir, sizeof ftp.dir},
-	{"ftp prefix", "/" MACH "/binary/sets", set_dir_bin, sizeof set_dir_bin},
-	{"ftp src prefix", "/source/sets", set_dir_src, sizeof set_dir_src},
+	{"ftp prefix", MACH "/binary/sets", set_dir_bin, sizeof set_dir_bin},
+	{"ftp src prefix", "source/sets", set_dir_src, sizeof set_dir_src},
 	{"ftp user", "ftp", ftp.user, sizeof ftp.user},
 	{"ftp pass", "", ftp.pass, sizeof ftp.pass},
 	{"ftp proxy", "", ftp.proxy, sizeof ftp.proxy},