process: Provide an abstraction to spawn child processes with redirected I/O
process: Port child process spawning to the Windows platform
process: Add a wrapper to invoke a command under the system default shell
updown: Use process abstraction to invoke updown script
ext-auth: Add an ext-auth plugin invoking an external authorization script

Original patch courtesy of Vyronas Tsingaras.
NEWS: Mention new ext-auth plugin
Merge branch 'ext-auth'

Integrates the ext-auth plugin by Vyronas Tsingaras. The new child process
abstraction simplifies implementation in both the new ext-auth and the existing
updown plugin, and makes them available on the Windows platform.
version bump to 5.2.1rc1
testing: Make TKM related build recipes future-proof

The tkm scenarios recently failed due to a segmentation fault on my host
because I had an old build of the tkm library already built in the build
directory.  Because the stamp file was not versioned the new release was
never checked out or built and charon-tkm was linked against the old
version causing a segmentation fault during key derivation.
testing: Make TNC scenarios agnostic to the actual Debian version

The scenarios will work with new or old base images as long as the version
in use is included as product in the master data (src/libimcv/imv/data.sql).
testing: Don't check for the actual number of SWID tags in PDP scenarios

The number of SWID tags varies depending on the base image, but lets
assume the number is in the hundreds.
vici: Fix message encoding type values in documentation
vici: Return a success result for the clear-creds command

Even if the command actually can't fail, this looks more aligned to similar
commands.
vici: Add a ruby gem providing a native vici interface
configure: Add global --enable-ruby-gems and --with-rubygemdir options

This provides the options to build and install ruby gems for components
providing them, such as vici.
vici: Use "gem"-assisted vici ruby gem building and installtion
WIP: vic ruby gem test