builder: index: Add checksum[sha512]=... option and deprecate template sigs.

We don't need to sign individual templates, provided the signed index
file contains the cryptographically secure checksums of the templates.
Since the index file cannot be interfered with and the checksums
cannot be spoofed, this is sufficient to ensure the integrity of the
templates.

Note the template signatures are now deprecated and will be removed in
libguestfs 1.26.  To avoid any backwards compatibility issues, I will
backport the relevant changes to the stable-1.24 branch.

Thanks Dennis Gilmore for pointing out that we could do this.
builder: When debugging, print curl command before running it.
builder: Allow multiple source paths to be specified.

Users can now specify multiple source paths, eg:

  virt-builder --source http://example.com/foo \
    --source http://example.com/bar

to get templates from multiple places.

There is still only one built-in path, but we can add more later.
./run: Disable timeouts in tests on RHEL 6.

Workaround for https://bugzilla.redhat.com/show_bug.cgi?id=1025269
builder: Add missing files to EXTRA_DIST.
Version 1.25.3.