HBSD: add 'Download only' option

Add option '-F' for download the latest update archive.
The option is able to resume an interrupted download.

Fix issues #57

 signaled by Shawn Webb.
Signed-off-by: Loic <loic.f@hardenedbsd.org>

usr.sbin/hbsd-update/hbsd-update

 #!/bin/sh
 #-
-# Copyright (c) 2015,2020 HardenedBSD
+# Copyright (c) 2015,2020,2021 HardenedBSD
 # Author: Shawn Webb <shawn.webb@hardenedbsd.org>
 #
 # This work originally sponsored by G2, Inc
@@ -43,6 +43,7 @@ dnssec_key="/usr/share/keys/hbsd-update/trusted/dnssec.key"
 revoke_dir="/usr/share/keys/hbsd-update/revoked"
 
 fetchonly=0
+downloadonly=0
 ignorever=0
 install_src=0
 integriforce=1
@@ -118,6 +119,7 @@ usage() {
 	debug_print "\t-c config\tUse a non-default config file"
 	debug_print "\t-d\t\tDo not use DNSSEC validation"
 	debug_print "\t-f\t\tFetch only"
+	debug_print "\t-F\t\tDownload only"
 	debug_print "\t-h\t\tShow this help screen"
 	debug_print "\t-I\t\tInteractively remove obsolete files"
 	debug_print "\t-i\t\tIgnore version check"
@@ -145,6 +147,10 @@ sigint_handler() {
 		destroybe=${1}
 	fi
 
+	if [ ${downloadonly} = 1 ]; then
+		exit 0
+	fi
+
 	debug_print "[-] Caught SIGINT. Cleaning up."
 
 	cleanup
@@ -1117,7 +1123,7 @@ main() {
 	local_kernel=""
 	no_kernel=0
 
-	while getopts '46dfhimnosBCDITUVb:c:j:K:k:r:Rt:u:v:' opt; do
+	while getopts '46dfFhimnosBCDITUVb:c:j:K:k:r:Rt:u:v:' opt; do
 		case "${opt}" in
 			4)
 				force_ipv4="yes"
@@ -1146,6 +1152,9 @@ main() {
 			f)
 				fetchonly=1
 				;;
+			F)
+				downloadonly=1
+				;;
 			i)
 				ignorever=1
 				;;
@@ -1248,6 +1257,13 @@ main() {
 		exit 1
 	fi
 
+	if [ ${downloadonly} = 1 ]; then
+		debug_print "[*] Download the latest update to the current folder."
+		${FETCH} ${net_flag} -o update-$(get_version).tar -arR \
+		${baseurl}/update-$(get_version).tar
+		exit 0
+	fi
+
 	check_version
 	res=${?}
 	if [ ${res} -gt 0 ]; then