Support reading build options from "config"

The Makefile will load the lines starting with "DINIT_" and define them as
preprocessor macros (if the "config" file is present). This allows for
build-time configuration of dinit.
Ensure /run/lock and /run/udev are created

Those are expected to be used by all the GNU/Linux distributions
out there in the wild.
Set the host name on initialization

Uses sethostname() to set the host name to the value stored in
"/etc/hostname" (if present).
Re-mount the root file system read-write

This is neded as first step of dinit_init_filesystem(), so directories for
mount points can be created as needed. This applies to root file systems
mounted as read-only by the kernel (either because that is the default, or
because the "ro" option has been passed in the command line).