userland-fetch: improvements for downloading and hashing

HASH_DIR support: search for an envvar HASH_DIR, a hashes/ directory in
the directory of --file, or a hashes/ the current directory. If found, look
inside the files in that directory for checksums in the format outputted by
sha256sum. Checksum files will be used if --hash is not provided.

partial downloads: any cancelled download leaves a .part file, which can
be continued later if the server supports it, or will be overwritten if not.

retrying: if there is a retryable network error, userland-fetch will try
to continue the download once or twice before quitting.

security tests: fail on unsecured, unhashed download with no signature.

Removed deprecated splithost and splittype commands.

Added -n/--need-hash option: userland-fetch will exit if a hash cannot be found
for the file.
Added -N/--need-sig option: userland-fetch will exit if a signature cannot be
found for the file, cannot be checked, or fails validation.

Added -g/--get-hashes option: userland-fetch will look for remote hash
files, and download them if there aren't any local copies.
Added -G/--get-sigs option: userland-fetch will look for remote
signatures and download them, overwriting local copies.
Added -c/--clobber-hash option: userland-fetch will replace files in
HASH_DIR with new files, if found (the same way as -g/--get-hashes).

If things get too strict, set the envvar ALLOW_UNVERIFIED_DOWNLOADS=yes
to skip some validation steps.