fdm_ptmx(), the FDM callback handler for ptmx data, is just as much in
the hot path as vt_from_slave(). It is also slightly more complicated
than a read() followed by a call to vt_from_slave().
As a result, some benchmarks performed significantly worse in a
partial PGO build than in a full PGO build, since fdm_ptmx() wasn’t
PGO:d.
To be able to feed data through fdm_ptmx(), we need to set up the
delayed rendering timer FDs, configure the timeout values, and provide
a readable FD it can read the VT data from.
The latter is done with a memory FD. This ensures *all* VT data is
loaded into memory before we feed it to the parser.
