[cdc_rsync] Detect remote architecture (#86)

Improves ServerArch so that it can detect the remote architecture by
running uname and checking %PROCESSOR_ARCHITECTURE%. So far, only
x64 Linux and x64 Windows are supported, but in the future it is easy
to add support for others, e.g. aarch64, as well.

Before the detection is run, the remote architecture is guessed first
based on the destination. For instance, if the destination directory
starts with "C:\", it pretty much means Windows. If cdc_rsync_server
exists and runs fine, there's no need for detection.

Since also PortManager depends on the remote architecture, it has to
be adjusted as well. So far, PortManager assumeed that "local" means
Windows and "remote" means Linux. This is no longer the case for
syncing to Windows devices, so this CL adds the necessary abstractions
to PortManager.

Also refactors ArchType into a separate class in common, since it is
used now from several places. It is also expanded to handle future
changes that add support for different processor architectures, e.g.
aarch64.

tests_common/BUILD

@@ -22,6 +22,7 @@ cc_binary(
     ],
     deps = [
         "//common:ansi_filter",
+        "//common:arch_type",
         "//common:buffer",
         "//common:dir_iter",
         "//common:file_watcher",