optimize & rmv from public API Perl_tmps_grow and related code

Previously in PUSH_EXTEND_MORTAL__SV_C, "PL_tmps_ix + 1" would execute
twice, once for the nonmutable if(>=), then again after the potential
tmps_grow call. tmps_grow has an unused return register/void proto, put it
to use by returning ix.  Also change tmps_grow to take the result of
"PL_tmps_ix + the constant (usually 1) or non-constant (EXTEND_MORTAL)".
This avoid having to put the constant twice in machine code, once for the
if test, 2nd time for extend length param for tmps_grow call. For
non-constant/EXTEND_MORTAL usage, it allows the C optimizer to have the
length var to go out of liveness sooner if possible. Also the var used for
the if(>=) test is more likely to be in a register than length var.
So "if test variable" is closer on hand to the CPU than length var. In some
cases, if non-const len var isn't used again, it becomes the "ix" variable
by having PL_tmps_ix added to it. Change sv_2mortal to return sv instead
of NULL to remove a unique branch/block of machine code that assigns 0 to
return variable (Visual C didn't figure out return sv == returned NULL,
not sv). See also [perl #121845].
Set PERL_MM_USE_DEFAULT in make_ext.pl

In the perl core, we shouldn’t be getting messages like this when
trying to build perl:

Should I do external tests?
These tests will fail if there is no internet connection or if a firewall
blocks or modifies some traffic.
[y/N] [n]

libnet’s Makefile.PL now does that.  To make it easier to maintain
such Makefiles.PL to work both on CPAN and in the core, set
PERL_MM_USE_DEFAULT temporarily in make_ext.pl when running the
Makefile.PL.
move dMY_CXT in dlutils.c closer to first use

By computing dMY_CXT right before its first and usually last use, my_cxt
pointer doesn't need to be carried across function calls in SaveError. This
frees a C stack auto and/or register from being saved and restored in
SaveError. On VC 2003 32 bits -O1 threaded, the machine code size in bytes
dropped from 0x72 to 0x70 after this patch.