io: update access mode handling in open function

Author: zoziha

doc/specs/stdlib_io.md

@@ -52,7 +52,8 @@ Experimental
 
 ### Description
 
-Returns the unit number of a file opened to read, to write, or to read and write. The file might be a text file or a binary file. All files are opened using a streamed access.
+Returns the unit number of a file opened to read, to write, or to read and write. The file might be a text file or a binary file.
+Text files are opened using a sequential access, while binary files are opened using a streamed access.
 
 ### Syntax
 

src/stdlib_io.fypp

@@ -460,14 +460,14 @@ contains
     select case (mode_(3:3))
     case('t')
       form_='formatted'
+      access_='sequential'
     case('b')
       form_='unformatted'
+      access_ = 'stream'
     case default
       call error_stop("Unsupported mode: "//mode_(3:3))
     end select
 
-    access_ = 'stream'
-
     if (present(iostat)) then
       open(newunit=u, file=filename, &
           action = action_, position = position_, status = status_, &