FreeBSD: only define B_FALSE/B_TRUE if NEED_SOLARIS_BOOLEAN is not set

If NEED_SOLARIS_BOOLEAN is defined we define an enum boolean_t, which
defines B_TRUE/B_FALSE as well. If we have both the define and the enum
things don't build (because that translates to
'enum { 0, 1 } boolean_t').

While here also remove an incorrect '#else'. With it in place we only
parse a section if the include guard is triggered. So we'd only use that
code if this file is included twice. This is clearly unintended, and
also means we don't get the 'boolean_t' definition. Fix this.

Reviewed-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Kristof Provost <kprovost@netgate.com>
Sponsored-By: Rubicon Communications, LLC ("Netgate")
Closes #13596