regcomp.sym: longj field is a boolean

regcomp.sym: longj field is a boolean

The comments could lead one to thinking one could specify any of the
argument fields that nodes can have.  But in fact, the value is a
boolean, 0 meaning to use the normal offset field of all regnodes; and 1
meaning to use the ARG field that some regnodes have.  If a regnode had
more than just the one argument field, the one that corresponds to that
would be used.

This commit enforces that, and changes regcomp.sym to not use  '2',
which is misleading.

It clarifies the comments about this and what '.' means in the flags
field