pgo: feed VT data through fdm_ptmx(), not vt_from_slave()

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.

terminal.c

@@ -198,7 +198,9 @@ fdm_ptmx_out(struct fdm *fdm, int fd, int events, void *data)
 static struct timespec last = {0};
 #endif
 
-static bool
+/* Externally visible, but not declared in terminal.h, to enable pgo
+ * to call this function directly */
+bool
 fdm_ptmx(struct fdm *fdm, int fd, int events, void *data)
 {
     struct terminal *term = data;