Fix an implementation inconsistency with the prop_*_send_syscall() and
prop_*_recv_syscall() functions from proplib(3). They now share the
same logic as the one from prop_*_send_ioctl() functions:
- returns an int
- 0 indicates "no error", otherwise returns the error number (and
set errno)

Many consumers of the prop_*_{ioctl, syscall} expect errno to be set
on error and use err() to display the error message. As such, ensures that
errno gets set before returning from these functions.

prop_*_send_syscall() functions returned a boolean, and now return an int.
Fix all call sites to use the new paradigm (only quota2 is affected in src).

As the prop_*_{send,recv}_syscall() API appeared in -current and is only
used by the recent quota2 code, I am not bumping the lib. The API change
only affects the prop_*_send_syscall() function (recv_syscall()s were
already used correctly), so ensure you are not mixing "old" -current
quota binaries with a new proplib(3) (or the other way around). This
change will be announced via a HEADS-UP and UPDATING.

Does not affect the kernel part of proplib.

Document the correct API in prop_array(3) and prop_dictionary(3).

Thanks to Francois Tigeot for noticing the API inconsistency and
reporting it on tech-kern@.

ok bouyer@.