Tell tags commands to use tags table file FILE.
FILE should be the name of a file created with the `etags' program.
A directory name is ok too; it means file TAGS in that directory.
Normally M-x visit-tags-table sets the global value of `tags-file-name'.
With a prefix arg, set the buffer-local value instead.
When you find a tag with M-., the buffer it finds the tag
in is given a local value of this variable which is the name of the tags
file the tag was in.
(fn FILE &optional LOCAL)