bootstrap: support checksum utils without a --status option

bootstrap: support checksum utils without a --status option

* build-aux/bootstrap: Only look for sha1sum if updating po files.
Add sha1 to the list of supported checksum utils since it's now
supported through adjustments below.
(update_po_files): Remove the use of --status
in a way that will suppress all error messages, but since this is
only used to minimize updates, it shouldn't cause an issue.
Exit early if there is a problem updating the po file checksums.
(find_tool): Remove the check for --version support as this
is optional as per commit 86186b17.  Don't even check for the
presence of the command as if that is needed, it's supported
through configuring prerequisites in bootstrap.conf.
Prompt that when a tool isn't found, one can define an environment
variable to add to the hardcoded search list.