Remove _map_libc symbol (re: 4c39a97)

It was only on by default for MVS (now z/OS, an IBM mainframe OS),
because apparently it doesn't allow overriding *any* libc functions
from shared libraries, so all AST functions had to be renamed. On
any system where we currently support shared libraries/dynamic
linking, this is not needed.

_map_libc could be optionally turned on by exporting
PACKAGE_OPTIONS=map-libc (that's what the features/options script
did) or by passing -D_map_libc in CCFLAGS. But I don't see any
actual use for this. Building with renaming all the functions was
broken for years and no one complained. I think we can do without
the maintenance load.

src/cmd/ksh93/data/builtins.c

@@ -47,7 +47,6 @@
 
 #undef	basename
 #undef	dirname
-#undef	mktemp		/* undo possible map-libc mktemp => _ast_mktemp */
 
 /*
  * IMPORTANT: The order of these struct members must be synchronous

src/lib/libast/Mamfile

@@ -133,11 +133,6 @@ make install virtual
 		exec - invoke_iffe %{<}
 	done
 
-	make FEATURE/options
-		makp features/options
-		exec - invoke_iffe %{<}
-	done
-
 	note * sig.h: AST replacement for <signal.h>
 	make FEATURE/sig
 		makp features/sig.sh
@@ -186,11 +181,6 @@ make install virtual
 	make ast_map.h
 		make FEATURE/map
 			make features/map.c
-				prev sig.h
-				prev FEATURE/api
-				prev FEATURE/eaccess
-				prev FEATURE/options
-				prev FEATURE/mmap
 				prev FEATURE/lib
 			done
 			exec - invoke_iffe %{<} -I.