Sync our DTrace infrastructure with c.h's definition of type bool.

Since commit d26a810e, we've defined bool as being either _Bool from
<stdbool.h>, or "unsigned char"; but that commit overlooked the fact
that probes.d has "#define bool char".  For consistency, make it say
"unsigned char" instead.  This should be strictly a cosmetic change,
but it seems best to be in sync.

Formally, in the now-normal case where we're using <stdbool.h>, it'd
be better to write "#define bool _Bool".  However, then we'd need
some build infrastructure to inject that configuration choice into
probes.d, and it doesn't seem worth the trouble.  We only use
<stdbool.h> if sizeof(_Bool) is 1, so having DTrace think that
bool parameters are "unsigned char" should be close enough.

Back-patch to v12 where d26a810e came in.

Discussion: https://postgr.es/m/CAA4eK1LmaKO7Du9M9Lo=kxGU8sB6aL8fa3sF6z6d5yYYVe3BuQ@mail.gmail.com