[perl #120903] perlcall cleanup

[perl #120903] perlcall cleanup

- linkify references to sections

- use EXTEND(SP, n) and PUSHs() instead of XPUSHs() where applicable
  and update prose to match

- add POPu, POPul and POPpbytex to the "complete list of POP macros"
  and clarify the documentation for some of the existing entries, and
  a note about side-effects

- add API documentation for POPu and POPul

- use ERRSV more efficiently

- approaches to thread-safety storage of SVs.

- minor updates