process::detail::descriptor non-owning implementation.

process::detail::std{in,out,err}_descriptor() implementation.
process::context: std{out,err}_data implementation.
bunsan/process/execute.hpp style fixes.
process::detail::descriptor::dup2(): min_fd -> new_fd.
process::unix::sync_execute::std_out_err_file test case style fix.
bunsan/process/execute.hpp style fixes.
process::unix test suite style fixes.
process::context::build_() comment fix.
process::context::build_() style fixes.
process::context::build_() bug fix.
process::context::build_() style fix.