doco stat fields

[Kevin Ryde]

Prompted by Stefan mentioning AC_STRUCT_ST_BLOCKS, a possible few
words about stat fields not always available.

        * posix.texi (File System): stat:rdev and stat:blocks can return #f,
        stat:blksize returns a sensible size if the field is not available.

Formatted text for ease of contemplation.  The second sentence of each
is new.

      - Scheme Procedure: stat:rdev st
          Device ID; this entry is defined only for character or block
          special files.  On some systems this field is not available
          at all, in which case `stat:rdev' returns `#f'.

      - Scheme Procedure: stat:blksize st
          The optimal block size for reading or writing the file, in
          bytes.  On some systems this field is not available, in which
          case `stat:blksize' returns a sensible suggested block size.

      - Scheme Procedure: stat:blocks st
          The amount of disk space that the file occupies measured in
          units of 512 byte blocks.  On some systems this field is not
          available, in which case `stat:blocks' returns `#f'.

posix.texi.stat.diff
Description: Text document