File Open Flags and Enforcement

[dthain]

Fix up the use of access flags for open files, in several places:
1 - User programs that call syscall_open are currently passing in magic integers to indicate various file flags. Instead, they should be using KERNEL_FLAGS_READ etc defined in kernel/types.h
2 - Drop FS_FLAGS_READ|WRITE in fs.h
3 - Remember the flags with which a kobject was opened, and reject attempts to write a file not opened with KERNEL_FLAG_WRITE, or to seek on a file not opened with KERNEL_FLAGS_RANDOM

[dthain]

@dsmith47 I'm happy to have you work on any of #230 #238 #246, but don't claim them all at once time, that tends to block up the queue. Claim one, work on it, make a PR, then claim another one...