#173 SHA crypt support

[Jan Engelhardt]

Webmin currently only offers crypt and MD5. Further algorithms like Blowfish/SHA are available on select systems using the standard crypt interface (not sure about what perl's Crypt::UnixCrypt::crypt would do) by means of

- $2a$06$%22s (bf, mandatory rounds selector, salt)
- $5$%16s$ (SHA-256+salt) and
- $6$%16s$ (SHA-512+salt).

In fact, the standard crypt interface may also support $1$%8s$ (MD5).