add challenge hook and minor fixes/improvements

Prevents an error if running for the first time in a different
directory.

letsencrypt.sh is mainly used with letsencrypt.org.

No code changes.

Shouldn't be necessary for output from `mktemp`, but better play it
safe.

big portability + readability fix: use openssl base64 instead of system base64, use shasum instead of shasum, added some comments

fix registration, certificate creation and minor fixes

source searches in $PATH which should not be necessary and might be
problematic.

printf is more portable and never interprets any escape characters.

Found by shellcheck.

Found by shellcheck.

Found by shellcheck.

Found by shellcheck.

Found by shellcheck.

We must strip the trailing newline with `echo -n` before we pass it to
perl to convert the hex to binary, not after.