Extend the callback table of UCOM to include a
"ucom_free" function pointer which is called when
all refs on a UCOM super structure is gone.
Implement various helper functions to handle
refcounting and draining on the UCOM super
structure.
Implement macro which can be used in device
drivers to avoid module unload before all
pending TTY references are gone.
The UCOM API is backwards compatible after this
change and device drivers require no changes
to function with this change. Only a recompilation
of UCOM device drivers is required. The FreeBSD
version has been bumped in that regard.
Discussed with: kib, ed
MFC after: 2 weeks
8f42c74844