Replace SHA-1 library with one that is clearly licensed.

Replace SHA-1 library with one that is clearly licensed.

The SHA-1 library that we used until now was taken from RFC 3174.  That
library has no clearly free license statement, only a license on the text
of the RFC.  This commit replaces this library with a modified version of
the code from the Apache Portable Runtime library from apr.apache.org,
which is licensed under the Apache 2.0 license, the same as the rest of
Open vSwitch.