embed.fnc: Remove inappropriate 'p' flags

These functions do not begin with 'Perl_'; currently this flag is
ignored here.
regen/embed.pl: Warn if have > 1 i, p, and s flags

These should be mutually exclusive
regen/embed.pl: Extract out duplicate code into a fcn
makedef.pl: Don't export inline fcns

An inline function is static to the file it is defined in.  But they can
be part of the public API if they are in #included header files and
defined in embed.fnc.  Normally such functions are written to the export
list on platforms that have this, but since they are static, you get a
linkage error.  This commit suppresses the writing of 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 inline.
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.