Turn AS_SHELL_SANITIZE into a for-Libtool-only wrapper.

Turn AS_SHELL_SANITIZE into a for-Libtool-only wrapper.

* lib/m4sugar/m4sh.m4 (_AS_DETECT_REQUIRED, _AS_DETECT_SUGGESTED): Remove
m4_require of _AS_DETECT_BETTER_SHELL.
(_AS_CLEANUP): Add it here.
(_AS_DETECT_BETTER_SHELL): Just expand the test instead of appending it
to _AS_CLEANUP.
(_AS_SHELL_SANITIZE): New name of the old AS_SHELL_SANITIZE macro.
(AS_SHELL_SANITIZE): New macro hacking around Libtool misuse.
(AS_PREPARE): Use _AS_SHELL_SANITIZE.
(AS_INIT): Add m4_provide of itself.