Actually free Process fd_map entries when a file is closed...

Actually free Process fd_map entries when a file is closed...
amazingly we never did that before.  Caused us to run out of
file descriptors in twolf.

sim/process.cc:
    Add free_fd() method to free closed target fd in simulator fd map.
    Rename open_fd() to alloc_fd() for symmetry with free_fd().
sim/process.hh:
    Add free_fd() method to free closed target fd in simulator fd map.
    Rename open_fd() to alloc_fd() for symmetry with free_fd().
    Crank up MAX_FD while we're at it.
sim/syscall_emul.cc:
    Call free_fd() on process when target closes a file.
sim/syscall_emul.hh:
    Process open_fd() renamed to alloc_fd().

--HG--
extra : convert_revision : d780f4ccfd5a0989230b0afbdbd276212b87550c