Not all tools executed by tasks support the '@argsfile' syntax for
shunting commandline arguments to a file. This means that if such
commands are shunted to a file early, he command will not work. On
windows the rc.exe command is such an example, but some tools on linux
have similar limitations. In the posix case, we artifically limit our
commandline size because it is difficult/variable to caluclate what the
actual limit is (it is partially dependent on environment size). This
could artifically cause commands to fail due to commandline length when
they otherwise wouldn't.
This patch fixes this issue by adding the 'allow_argsfile' flag to the
task. This way certain task instances will be able to specify if they
are compatible with the '@argsfile' syntax or not.
