Clean up the MSDOS port

Clean up the MSDOS port

* src/msdos.h (tcdrain): Redirect to '_dos_commit'.
(openat, fchmodat, futimens, utimensat): Add prototypes.

* msdos/sed1v2.inp (MAKE_PDUMPER_FINGERPRINT): Fix indentation, so
that Make won't consider this line a command.
($(etc)/DOC): Chdir back to ../src, since "make-docfile -d" leaves
us in a wrong directory.
* msdos/sedlibmk.inp (GL_GNULIB_GETRANDOM, GL_GNULIB_MEMMEM)
(GL_GNULIB_SIGDESCR_NP): Define to 1, to get the prototypes from
Gnulib headers.