Automatically OSS sign binaries during build.

Automatically OSS sign (delay sign and then set the "signed" CLI
header flag) binaries during build.  The "SkipSigning" property can
be set by an individual project to prevent signing that assembly, and
we do this for our build tools assemblies.  The SkipSigning property
can also be set on the command line to prevent signing all assemblies.
Merge branch 'enable-signing'