Use the defines from pcb.h over the ones from regnum.h for this 'C'
code. In theory, the ones from regnum.h should be used only for
assembler code.
Make it possible to include this file in assembler .S sources.
Respect the enoent_ok flag if reporting error for holding an non-existing
snapshot.

Related illumos ZFS issue:
  3699 zfs hold or release of a non-existent snapshot does not output error

Reported by:	Steven Hartland <smh@FreeBSD.org>
MFC after:	3 days
revert r248644 because of the regression for usdt probes

USDT probes are advertised to kernel by initialization code with
atrribute((constructor))).  It seems that on Solaris the .init-ish code
of the main object is executed before RD_PREINIT point is hit.  On
FreeBSD that is not the case.  And because on FreeBSD there is no other
well-defined point between RD_PREINIT and main() we have to parse a
DTrace script when main is hit, for time being.

A footnote: currently we actually post RD_POSTINIT event, but that's a
bug because the event is triggered by hitting r_debug_state which
happens before any init code is executed.

Reported by:	markj