From the newsgroup: it was possible to saddle, mount, and ride on a
sleeping jabberwork without it ever waking up. Movement was checking for
timed sleep (!mon->mcanmove, set when mon->mfrozen contains a timer count
for either sleep or paralysis) but not indefinite sleep (mon->msleeping).
This moves the checking into its own routine which handles both types.
And it gives monsters a chance to wake up when they get saddled or mounted.
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