rv2hv does not use its TARG

rv2hv has had a TARG since perl 5.000, but it has not used it since
hv_scalar was added in perl-5.8.0-3008-ga3bcc51.

This commit removes it, saving a tiny bit of space in the pad.
pp_hot.c:pp_rv2av: Remove superfluous SPAGAIN

Commit perl-5.8.0-9908-g5e17dd7, in March of 2007, added a SPAGAIN
here to account for the stack shifting when hv_scalar calls tied
hashes’ SCALAR method.

It was never necessary, because commit perl-5.8.0-3008-ga3bcc51,
which added hv_scalar and magic_scalarpack in December of 2003, made
magic_scalarpack push a new stack, protecting the old one.
Propagate lvalue context to && and ||

I did this for the last statement in an lvalue sub (and the argument
to ‘return’ in an lvalue sub) in commit 2ec7f6f2428.

It turns out that it needs to apply in other lvaluish contexts, too:

$ ./perl -Ilib -le 'for($a||$b){$_++} print $b'
1
$ ./perl -Ilib -le 'for(pos $a || pos $b){$_++} print pos $b'
Modification of a read-only value attempted at -e line 1.

If I can assign to $b through this construct, then why not pos?