Be more careful when recreating single-precision (float) argument to service
precise exceptions, as the actual data the FPU gives us is a 35-bit number,
with the exponent sign-extended to the double-precision exponent width.

Make sure we shrink it and fetch the remaining three low bits of mantissa from
the LS registers.
Warnings. I haz them.
Memory leak in error path of the day, from clang via dhill@bitrig;
ok dhill@bitrig
Various small typographic fixes for mman.h manual pages:

Use .Fn instead of .Nm as appropriate
Use .In for include lines
Use .Rv -std where possible
Use .Xr to refer to functions from other manual pages
Remove extraneous sys/types.h include

More substantive changes to follow.

Discussed with schwarze