fcntl(F_SETFL) and setsockopt use translate() instead of translate_writable_vfd()

[perbu]

lib/tinykvm/linux/system_calls.cpp:1828 (SYS_fcntl, F_SETFL) and
:1133 (SYS_setsockopt).

Both are mutating operations on a host fd, but both resolve the vfd with the
plain translate() rather than translate_writable_vfd(). The neighbouring
F_SETLK64 case at :1855 uses translate_writable_vfd(), which is the
intended idiom for a write path — the local variable at :1828 is even already
named writable_fd.

Consequence worth noting for the fork deployment: forks share the master's
real_fd (fds.cpp:303,331), and reset_to() does not restore file status
flags, so a flag set through this path persists across a reset.

Fix: use translate_writable_vfd() in both places.

Found by an internal audit of the host-side Linux emulation. A fix PR with a
regression test follows.

[perbu]

Fix in #108.