fdcopy(): properly update fd_freefile when skipping non-copyable fd

[kib]

Summary

fd_freefile is maintained to fulfill the POSIX guarantee that the lowest
possible file descriptor is allocated on open.  If we skip copying an
fd, it is a possible candidate for the lowest index.

First, the file descriptor installed with a kqueue created
without KQUEUE_CPONFORK missed this handling [*].

Second, fd_freefile should be updated only when the current fd index is
below it.  A skipped fd above fd_freefile could raise it past a free slot.

PR:     300234 [*]
Reported by:    Andrea Venturoli <[e-mail]>
Fixes:  1265516c5c61 ("struct file: add DFLAG_FORK, indicate copying on fork")
Fixes:  809f3121be66 ("fd: assign fd_freefile early when copying")