Add decrypt_bytes() and verify_bytes().

Add decrypt_bytes() and verify_bytes().

Create the encrypted test input with:

  $ echo 'Success!' | gpg --no-verbose --quiet --batch --output -
      --armor --textmode --encrypt --always-trust
      --recipient pgp-mime@invalid.com

Create the signed and encrypted test input with:

  $ echo 'Success!' | gpg --no-verbose --quiet --batch --output -
      --armor --textmode --sign --encrypt --always-trust
      --local-user pgp-mime@invalid.com --recipient pgp-mime@invalid.com

Created the detached signature test input with:

  $ echo 'Success!' | gpg --no-verbose --quiet --batch --output -
      --armor --textmode --detach-sign --always-trust
      --local-user pgp-mime@invalid.com

Verification with a detached signature is the tricky bit.  We are
piping the signed data in via stdout.  To avoid opening a temporary
file, we need to pipe the signature in through another pipe.  The new
`thread_pipe()` function opens that pipe, and spawns a thread writing
the signature data, and the `--enable-special-filenames` option lets
us specify the read-descriptor with the `-&n` syntax.

The threading avoids deadlocking with `execute()`'s `communicate()`
call, and makes cleanup of the write-descriptor easier.