Handle RW case for open

[krs85]

Seems like the pash benchmarks like to open files for reading and writing. Surprisingly, I haven't run into anything before this that does this. But regardless, it needs to be handled.

First thought is it basically is open for reading + open for writing. So we need to add RW as a mode for SyscallEvent::Open() event. And then assign the proper preconditions and postconditions.

• Replace OFlag in SyscallEvent::Open() with a custom enum OpenMode { Read, Write, Both }
• Add another parameter to SyscallEvent::Open() : offset mode. This can be None if read only. Again, we can make a custom enum. Safer with less cases.
• handle_open()
• generate_open_syscall_event()
• generate_preconditions()
• generate_postconditions()
• copying to cache
• copying from the cache
• in the case where no offset mode (O_APPEND or O_TRUNC) is provided, the default behavior is to start writing at the beginning of the file, overwriting whatever is there. Yuck! But for our purposes, we treat this the same as an O_APPEND, conservatively treating the entire starting contents as a precondition.

[krs85]

Don't forget stdout :-)