Fixed Bug #66833 Default digest algo is still MD5

Switch to SHA1, which match internal openssl hardcoded algo.

In most case, won't even be noticed
- priority on user input (default_md)
- fallback on system config
- fallback on this default value

Recent system reject MD5 digest, noticed in bug36732.phpt failure.

While SHA1 is better than MD5, SHA256 is recommenced,
and defined as default algo in provided configuration on
recent system (Fedora 21, RHEL-7, ...). But the idea is to
keep in sync with openssl internal value for PHP internal value.
NEWS