core: better debugging of serial port open/close

Some devices don't interact well with the option drivr or the usb-serial
layer; they don't respond to the USB data requests and thus data never
gets written to that port.  When close(2) is called, that data is still
pending and so the tty layer waits 30 seconds before returning from
the close.  This is the 'closing_wait' value, which unfortunately is
not able to be modified by ModemManager because most serial drivers
for 3G devices don't implement the .ioctl handler or its TCIOSSERIAL
option to change closing_wait.

Print out open/close timestamps to help debug this issue and get a
list of modems that have this problem.

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