gdbsupport: move include/gdb/fileio.h contents to fileio.h

gdbsupport: move include/gdb/fileio.h contents to fileio.h

I don't see why include/gdb/fileio.h is placed there.  It's not
installed by "make install", and it's not included by anything outside
of gdb/gdbserver/gdbsupport.

Move its content back to gdbsupport/fileio.h.  I have omitted the bits
inside an `#if 0`, since it's obviously not used, as well as the
"limits" constants, which are also unused.

Change-Id: I6fbc2ea10fbe4cfcf15f9f76006b31b99c20e5a9