Make sure we check all the functions' return values. Minor API changes to simplify this. Checked using a patched clang that adds attribute(warn_unused_result) to all functions. This commit fixes several error handling mistakes.

NFS cache flushing updates. --HG-- branch : HEAD

NFS cache flushing APIs changed and backend implements it a bit differently. Hopefully this works now more correctly. --HG-- branch : HEAD

nfs_flush_attr_cache() doesn't take flush_dir paramter anymore. Instead the directory is always flushed now. --HG-- branch : HEAD

nfs_safe_link(): Support linking files without original link count=1. Use it when copying maildir files, but only if mail_nfs_storage=yes. --HG-- branch : HEAD

Added nfs_safe_link(). --HG-- branch : HEAD

NFS attribute cache flushing fixes. nfs_flush_attr_cache() takes now flush_dir parameter and nfs_flush_attr_cache_fd() returns FALSE if file handle is already stale. --HG-- branch : HEAD

Changed .h ifdef/defines to use <NAME>_H format. --HG-- branch : HEAD

Added nfs_flush_attr_cache() and nfs_flush_read_cache(). --HG-- branch : HEAD

Added nfs_safe_lstat() --HG-- branch : HEAD

Include <sys/stat.h> before using struct stat, because some systems may use a macro to define struct stat to something else. --HG-- branch : HEAD

Added nfs_safe_stat() to transparently work around ESTALE with stat(). --HG-- branch : HEAD

Renamed safe-open.* to nfs-workarounds.*, safe_open() to nfs_safe_open() and added a new global NFS_ESTALE_RETRY_COUNT which everyone uses instead of defining their own. --HG-- branch : HEAD