proc: Drop some mach_port_destroy () uses

mach_port_destroy () is a dangerous API that has to be used with extreme care.
Namely, it destroys not one user reference, but *all* user references that a
task has for a port name. Different parts of a program may all keep separate
references on a port without coordinating it with each other (which is the
whole idea behind reference counting). If one part of a program decides to
destroy a port with mach_port_destroy () without informing others, others may
still believe they hold a reference and will continue to use the name as if it
still refered to the now-destroyed port right. This consitutes a port
use-after-free, even if their use is also deallocating their reference.

In the particular case of the proc server, this manifested itself as
S_proc_reassign () destroying all user references to the task port right before
the task port right is deallocated again in the dead-name notification handler.

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