gv.h: Add a gp_flags field

This is for flags that have to be shared with all GVs that have the
same GP pointer.
Skip no-common-vars optimisation for aliases

The ‘no common vars’ optimisation allows perl to copy the values
straight from the rhs to the lhs in a list assignment.

In ($a,$b) = ($c,$d), that means $c gets assigned to $a,
then $d to $b.

If the same variable occurs on both sides of the expression
(($a,$b)=($b,$a)), then it is necessary to make temporary copies of
the variables on the rhs, before assigning them to the left.

If some variables have been aliased to others, then the common vars
detection can be fooled:

    *x = *y;
    $x = 3;
    ($x, $z) = (1, $y);

That assigns 1 to $x, and then goes to assign $y to $z, but $y is
the same as $x, which has just been clobbered.  So 1 gets assigned
instead of 3.

This commit solves this by recording in each typeglob whether the sca-
lar is an alias of a scalar from elsewhere.

If such a glob is encountered, then the entire expression is ‘tainted’
such that list assignments will assume there might be common vars.
Dump GPFLAGS
toke.c: Fix bad read in S_scan_heredoc

If the first character of a source line matched the first character
of the here-doc terminator, then we did a memEQ with the length of
the terminator, without regard to the length of the line, which
could be shorter.

This bug goes all the way back to perl 3.000, which introduced here-
docs.  Line 1802 of toke.c in perl 3.000 is:

		    if (*s == term && bcmp(s,tokenbuf,len) == 0) {

and contains the same bug.

So this bug is twenty-five years old.