Handle @file syntax on Windows correctly (#392)

Handle @file syntax on Windows correctly (#392)

* add length return to win32argvtos()

this is useful for determining the length of the generated argument string

* correctly handle @file syntax on Windows

the @file syntax means that the process reads command arguments from the
specified file. this is commonly used in order to shorten commands which
would otherwise be longer than the maximum length limit: many build systems
do this in all cases to avoid hitting this limit.

when a command exceeds 8192 characters on on Windows, ccache now writes
the parsed/modified arguments to a tmpfile and then runs the command using
that tmpfile with @tmpfile in order to preserve this mechanism and avoid hitting
the length limit

fixes #95