Remove redundant SPAGAIN & PUTBACK after PUSHSTACKi().

PUSHSTACKi() calls SWITCHSTACK(), which sets PL_stack_sp and sp like this:

    sp = PL_stack_sp = PL_stack_base + AvFILLp(t)

Hence after PUSHSTACKi() both are identical, so use of SPAGAIN or PUTBACK
to assign one to the other is redundant.

The use of SPAGAIN in encoding.xs and via.xs was added with commit
24f59afc531955e5 (April 2002) which added the use of PUSHSTACKi(). It feels
like cargo-cult.

The use of PUTBACK in Perl_amagic_call() predates the introduction of nested
stacks and PUSHSTACKi() in commit e336de0d01f30cc4 (April 1998). It dates from
perl 5.000, but it's not clear that it was ever needed, as the code in
question looked like this, and nothing could have moved the stack between
the dSP and PUTBACK:

    dSP;
    BINOP myop;
    SV* res;

    Zero(&myop, 1, BINOP);
    myop.op_last = (OP *) &myop;
    myop.op_next = Nullop;
    myop.op_flags = OPf_KNOW|OPf_STACKED;

    ENTER;
    SAVESPTR(op);
    op = (OP *) &myop;
    PUTBACK;

The PUTBACK and SPAGAIN in Perl_require_pv() were added by commit
d3acc0f7e5197310 (June 1998) which also added the PUSHSTACKi(). They have
both been redundant since they were added.
S_process_special_blocks() should use a new stack for BEGIN blocks.

This avoids the stack moving underneath anything that directly or indirectly
calls Perl_load_module().
Perl_load_module() no longer moves the current stack, so no need to save it.
No need to wrap calls to Perl_load_module() in ENTER/LEAVE

As of commit 53a7735b62aee146 (May 2007) Perl_vload_module() wraps its call
to Perl_utilize() with ENTER/LEAVE, so there's no longer a need for callers
of Perl_load_module() to also wrap with ENTER/LEAVE.