vfs_fruit: rework struct adouble API

vfs_fruit: rework struct adouble API

o factor out ad_open(), opens an fd to be used with IO on the metadata
  streams

o rename ad_write() to ad_set() as this aligns nicely with the existing
  ad_get. This is the pathname based version used to solely set metadata
  on a file, there's also a handle based version:

o add ad_fset(), a handle based version that can be used to set metadata
  and to update the AppleDouble header in a ._ AppleDouble file

o remove fruit_fsp_recheck(), looking at it more closely, it is not
  needed

This commit *compiles*, but all callers in the VFS ops must now be
updated to use the new semantics, this comes next...

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12427

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Uri Simchoni <uri@samba.org>