Hide function prototyes from unauthorized callers

0351a62 extended hiding private
functions from callers into the gcc world.

Some functions are allowed only in extensions; so can not be marked as
hidden; this commit discourages their use however, by hiding their
prototypes to all but the core and extensions.

It turns out that three functions were being used in modules we ship
with that were marked as extensions-only; so they had to be made
globally accessible.

Author: khwilliamson

embed.fnc

@@ -1307,7 +1307,7 @@ ERXp	|SV *	|get_prop_definition					\
 ERTXp	|const char * const *|get_prop_values				\
 				|const int table_index
 : Used by SvRX and SvRXOK
-EXopx	|REGEXP *|get_re_arg	|NULLOK SV *sv
+Xopx	|REGEXP *|get_re_arg	|NULLOK SV *sv
 AOdp	|SV *	|get_sv 	|NN const char *name			\
 				|I32 flags
 CRipx	|MGVTBL *|get_vtbl	|int vtbl_id
@@ -3687,9 +3687,9 @@ Cp	|UV	|_to_utf8_upper_flags					\
 				|NULLOK STRLEN *lenp			\
 				|bool flags
 
-EXop	|bool	|try_amagic_bin |int method				\
+Xop	|bool	|try_amagic_bin |int method				\
 				|int flags
-EXop	|bool	|try_amagic_un	|int method				\
+Xop	|bool	|try_amagic_un	|int method				\
 				|int flags
 ARTdp	|char * |uiv_2buf	|NN char * const buf			\
 				|const IV iv				\