Make pos(@array) and pos(%hash) into errors

Make pos(@array) and pos(%hash) into errors

Currently pos has an effective prototype of (;\[$@%*]), and what it
does is rather interesting.

First, it produces a strange uninitialized warning:

$ ./perl -Ilib -we 'pos my @a = 3'
Use of uninitialized value within @a in scalar assignment at -e line 1.

There is no uninitialized value here.  The value ‘within @a’ is actu-
ally @a itself.  The code that produces the error message was written
under the (perfectly logical) assumption that an array would never be
passed to report_uninit().

Secondly, it adds pos magic to the array itself:

$ ./perl -Ilib -e 'pos @a = 3; use Devel::Peek; Dump \@a'
SV = IV(0x8039fc) at 0x803a00
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x825b90
  SV = PVAV(0x804a04) at 0x825b90
    REFCNT = 2
    FLAGS = (SMG)
    MAGIC = 0x30cb20
      MG_VIRTUAL = &PL_vtbl_mglob
      MG_TYPE = PERL_MAGIC_regex_global(g)
    ARRAY = 0x0
    FILL = -1
    MAX = -1
    ARYLEN = 0x0
    FLAGS = (REAL)

This magic can never be used, as @a =~ /foo/g is equivalent to
scalar(@a) =~ /foo/g, and scalar(@a) returns a scalar containing the
length of the array, not the array itself.

This seems clearly a mistake.

pos forces lvalue context on its argument, making pos(3) a compile-
time error.

Internally, the main distinction between \$ (scalar lvalue) and
\[$@%*] (scalar lvalue, or some other type) prototypes is that the
function S_scalar_mod_type returns true for functions with the former,
but false for functions with the latter.  (Tangentially, \[$@%*] and
\[$@%&*] are distinguished by the special-casing in op_lvalue_flags
under case OP_ENTERSUB.)

S_scalar_mod_type returns false for pos.  I think it should return
true.  That is what this commit does, resulting in consistency
with read():

$ ./perl -Ilib -we 'read($1, @2, $3)'
Can't modify array dereference in read at -e line 1, near "$3)
"
Execution of -e aborted due to compilation errors.
$ ./perl -Ilib -we 'pos(@2)'
Can't modify array dereference in match position at -e line 1, near "@2)
"
Execution of -e aborted due to compilation errors.

Except when it comes to globs, since read refuses *foo for its second
argument, but pos(*foo) has always Just Worked, so there is no reason
to forbid it.

So, now, pos has an effective prototype of (;\[$*]).