Further to pull #49 this is the result of an audit of the use of perl
stack macros. There were several cases where PUTBACK was being called
where the stack pointer could be out-of-date.
Also some misc. cleanup where the macros were used needlessly.
This removes the calls to Perl_get_context() that get automatically
added to XS code for ancient source code compatibility reasons.
The result is about a ~60K size reduction in the binary (based on
comparing two 64-bit stripped irssi binaries compiled
--with-perl-staticlib).
