bsd-user: implement path searching

bsd-user: implement path searching

Use the PATH to find the executable given a bare argument. We need to do
this so we can implement mixing native and emulated binaries (e.g.,
execing a x86 native binary from an emulated arm binary to optimize
parts of the build). By finding the binary, we will know how to exec it.

Signed-off-by: Stacey Son <sson@FreeBSD.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>