Re-visit SHA-1 code

Re-visit SHA-1 code

- Fixed build failures when configured with --enable-ssse3-sha1.

- Actually calculate the message digest correctly in the SSSE3-optimized
  version!  The Intel code just does block transformations, not arbitrary
  updates; the previous code did not reflect this.

- Use an appropriate fallback when the CPU does not support SSSE3
  instructions; don't just call abort()!

- Improve sha1_update() and sha1_final().  They should now be slightly
  faster, as well as easier to understand.

- Use beXX_to_cpu() and cpu_to_beXX() macros instead of hard-coding
  endian conversions.