gdbsupport: move fileio_errno_to_host to fileio.{h,cc} and rename

gdb_bfd.c and remote.c contain identical implementations of a
fileio_error -> errno function.  Factor that out to
gdbsupport/fileio.{h,cc}.

Rename it fileio_error_to_host, for symmetry with host_to_fileio_error.

Change-Id: Ib9b8807683de2f809c94a5303e708acc2251a0df