Enable loading alternative plugins and add executable check

Plugin loading code is reworked so Ark attempts other plugins, if
highest priority plugin fails. For CliPlugins, Ark now also searches for
executables (as specified in ParameterList in the individual plugin) in
standard path after the plugin has been loaded and if they are not found
moves on to next plugin in priority list. Read-write plugins require all
four types of executables to be found (ListProgram, ExtractProgram,
AddProgram and DeleteProgram), while read-only plugins require only
ListProgram and ExtractProgram.

This paves the way for setting cli7z priority higher than clizip to
improve unicode support for zip archives. If p7zip is not installed by
distributions, Ark will now automatically fall back to using the more
widely installed  clizip.

CCBUG: 221021