New API: write for creating files with fixed content (RHBZ#501889).

New API: write for creating files with fixed content (RHBZ#501889).

The guestfs_write call can be used to create small files with
arbitrary 8 bit content, including \0 bytes.

This replaces and deprecates write-file, which cannot be modified
to use BufferIn because of an unfortunate choice in the ABI: the
size parameter to write-file, if zero, means that the daemon tries
to calculate the length of the buffer using strlen.  However this
fails if we pass a zero-length buffer using BufferIn because then
the daemon tries to do strlen on a (really) zero length buffer, not
even containing a terminating \0 character, thus segfaulting.