Write current region into specified file.
When called from a program, requires three arguments:
START, END and FILENAME. START and END are normally buffer positions
specifying the part of the buffer to write.
If START is nil, that means to use the entire buffer contents.
If START is a string, then output that string to the file
instead of any buffer contents; END is ignored.
Optional fourth argument APPEND if non-nil means
append to existing file contents (if any). If it is an integer,
seek to that offset in the file before writing.
Optional fifth argument VISIT, if t or a string, means
set the last-save-file-modtime of buffer to this file's modtime
and mark buffer not modified.
If VISIT is a string, it is a second file name;
the output goes to FILENAME, but the buffer is marked as visiting VISIT.
VISIT is also the file name to lock and unlock for clash detection.
If VISIT is neither t nor nil nor a string,
that means do not display the "Wrote file" message.
The optional sixth arg LOCKNAME, if non-nil, specifies the name to
use for locking and unlocking, overriding FILENAME and VISIT.
The optional seventh arg MUSTBENEW, if non-nil, insists on a check
for an existing file with the same name. If MUSTBENEW is `excl',
that means to get an error if the file already exists; never overwrite.
If MUSTBENEW is neither nil nor `excl', that means ask for
confirmation before overwriting, but do go ahead and overwrite the file
if the user confirms.
This does code conversion according to the value of
`coding-system-for-write', `buffer-file-coding-system', or
`file-coding-system-alist', and sets the variable
`last-coding-system-used' to the coding system actually used.
This calls `write-region-annotate-functions' at the start, and
`write-region-post-annotation-function' at the end.
(fn START END FILENAME &optional APPEND VISIT LOCKNAME MUSTBENEW)