Fix Posix-conformance bugs re use of { command in sed scripts,

Fix Posix-conformance bugs re use of { command in sed scripts,
and improve the sed-related documentation a bit.
* doc/autoconf.texi (Installation Directory Variables): Use
our own style advice re 's,a,b,' versus 's|a|b|'.  Use "Sed"
rather than "sed" when talking about Sed in general.
(Particular Programs): Likewise.
(Coding Style): y is like s with respect to / and ,.
(Limitations of Usual Tools): Document the weird restrictions
that Posix has about { }.  Use better quoting.
* lib/autoconf/status.m4 (_AC_OUTPUT_FILES_PREPARE, _AC_OUTPUT_HEADER):
Rewrite to conform to Posix rules about { } in sed scripts.
* lib/m4sugar/m4sh.m4 (AS_DIRNAME_SED, AS_BASENAME_SED): Likewise.
* tests/foreign.at (Libtool): Likewise.
* tests/semantics.at (AC_CHECK_PROG & AC_CHECK_PROGS):
Use our own style advice re 's,a,b,' versus 's|a|b|'.