Update fd_freefile when filtering/closing kqueue descriptors in fdcopy().

Update fd_freefile when filtering/closing kqueue descriptors in fdcopy().

Prior to r1.153 of kern_descrip.c, the kqueue descriptors were removed
using fdremove(), which reset fd_freefile as appropriate. The new code
simply avoids adding the descriptor to the new table, however this means
that fd_freefile can be left with an incorrect value, resulting in a file
descriptor allocation "hole". Restore the previous behavour by lowering
fd_freefile as appropriate when dropping descriptors.

Issue found via golang regress tests.

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