contrib: FreeBSD compatibility in install_db4.sh

Unfortunately, FreeBSD uses yet another syntax for `sha256`.

Support FreeBSD's syntax too. Using `uname` is a bit of a hack but it
works and I found no way to distinguish the two.

contrib/install_db4.sh

@@ -31,7 +31,11 @@ sha256_check() {
   if check_exists sha256sum; then
     echo "${1}  ${2}" | sha256sum -c
   elif check_exists sha256; then
+    if [ "$(uname)" = "FreeBSD" ]; then
+      sha256 -c "${1}" "${2}"
+    else
       echo "${1}  ${2}" | sha256 -c
+    fi
   else
     echo "${1}  ${2}" | shasum -a 256 -c
   fi