check readonly by trying to open the file for writing...

 instead of mch_access, to avoid a possible race condition
move check_file_readonly() into /src/os/fs.c and rename it
move filewritable() into /src/os/fs.c and rename it
remove mch_access macro as it is only used in fs.c
cleaned up the readonly check in readfile()
added doxygen documentation for os_file_is_{readonly, writable}