In S_regatom, declare parse_start when RE_TRACK_PATTERN_OFFSETS is defined.

Commit 779fedd7c3021f01 (March 2013) moved code which unconditionally used
parse_start into another block. Hence the variable is now only needed when
RE_TRACK_PATTERN_OFFSETS is defined, so wrap its declaration in #ifdef/#endif
to avoid C compiler warnings.
In regcomp.c, Set_Node_Cur_Length() uses parse_start, so explicitly pass it.

The macro Set_Node_Cur_Length() had been referring to the variable
parse_start within its body. This somewhat secret reference is potentially
risky, as it was always taking a parameter node, hence one might assume that
that was all that it used, and change the value stored in parse_start.
(Specifically, one place that assigns RExC_parse - 1 to parse_start, and later
uses parse_start + 1, which looks like an obvious cleanup candidate)

So make parse_start an explicit parameter.

Also, eliminate the never-used macro Set_Cur_Node_Length. This was added as
part of commit fac927409d5ddf11 (April 2001) but never used.