main/ca-certificates: add copy of c_rehash tool

We want to avoid ca-certificates dependency for openssl tool for
two reasons:
- it is very large to be mandatory dependency
- it disallows user choosing if openssl or libressl package
  should be installed

Neither the openssl c_rehash nor libressl certhash can be any longer
compiled standalone. So include the original c_rehash.c code that
was submitted for openssl inclusion that can be built standalone.