embed.fnc: Remove inappropriate 'p' flags

These functions do not begin with 'Perl_'; currently this flag is
ignored here.
regen/embed.pl: Make error if have both p and s flags

This moves the current test for having both i and s to the new place
regen/embed.pl: Extract out duplicate code into a fcn
regen/embed.pl: Allow inline functions to have 'Perl_'

Inline functions are static to the file they are embedded in, so should
have a 'S_' prefix.  However if that file is a header file that
is #included and is listed in embed.fnc, makedef.pl will automatically
create a def list for Win32 that has a "Perl_" prefix.  This commit
allows the "p" flag to override the normal "S_" prefix for inline
functions.
Change name 'av_top' to 'av_top_index'

 In using the av_top() function created in a recent commit, I found
 myself being confused, and thinking it meant the top element of the
 array, whereas it really means the index of the top element of that
 array.  Since the new name has not appeared in a stable release, it can
 be changed, without remorse, to include 'index' in it.
Inline av_top_index()

This function is just an assert and a macro call.  Avoid the function
call overhead by making it linline.
Add av_tindex() synonym for av_top_index()

The latter is a somewhat less clumsy name.  The old one is provided a a
very clear name; the new one as a somewhat slangy version
Change pods to not refer to av_len()

This name for the function is misleading; don't encourage its use.