lib: don't use foo_real()-style symbols in a header file While there is precedent for symbols getting _real suffix to hide implementation details, all the existing symbols were static until the recent i_fd_close*() changes. This commit makes this true again.

lib: move i_fd_close*() to fd-util.[ch]

lib: move fd_close_maybe_stdio() to fd-util.[ch]

lib: move fd_set_nonblock() to fd-util.[ch]

lib: move fd-close-on-exec.[ch] code into fd-util.[ch]

lib: always include fd-close-on-exec.h & fd-set-nonblock.h via fd-util.h