More file operations and permissions

• shortened json keys (url,token,text,old,type) -> (a,b,c,o,t)
• include read_access flag in datalink (r)
• include mapping flag in datalink (m)
• new view datalink.access with triggers to update datalink.directory.diracl
• new function datalink.has_file_privilege() to enquire about file privileges
• initial datalink.insight table for future read access management
• stricter checks for file paths in dlvalue()
• added substr, instr, fileexists, filegetname and getlength bfile compatibility functions
• you can specify ranges in read_text and read_lines
• check for SELECT privilege in read_text and read_lines
• check for DELETE privilege in dl_datalink_ref