libc: Use musl's optimized strchr and strchrnul

Parentheses added to HASZERO macro to avoid a GCC warning, and formatted
with clang-format as we have adopted these and don't consider them
'contrib' code.

Obtained from: musl (snapshot at commit 4d0a82170a25)
Reviewed by: kib (libc integration), mjg (both earlier)
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17630