common: copy md5 implementation to iputils project

This allows avoiding linking to crypto libraries, or expect kernel api to
provide md5 functionality.  Notice that earlier use of kernel api could
result to binary zero md5 values being used, that is not how RFC4620 tells
subject names should be used.  See bug report for details.

Reported-by: Noah Meyerhans <[email protected]>
Addresses: iputils#239
Reference: https://tools.ietf.org/html/rfc4620
Reviewed-by: Petr Vorel <[email protected]>
Signed-off-by: Sami Kerola <[email protected]>