replace PL_doextract with better kinds of variable

replace PL_doextract with better kinds of variable

PL_doextract had two unrelated jobs, neither best served by an interpreter
global variable.  The first was to track the -x command-line switch.
That is replaced with a local variable in S_parse_body().  The second
was to track whether the lexer is in the middle of a =pod section.
That is replaced with an element in PL_parser.