do not require leading . for rl color prefix etension; fix for isearch in single-byte locales; next set of doc updates (SIGNALS); add warning for invalid job id; allow function names to be non-identifiers in posix mode

tests/errors.tests

@@ -388,10 +388,8 @@ ${THIS_SH} -o posix -c '. /nosuchfile ; echo after source' sh
 # but trap specifying a bad signal nunber is non-fatal
 ${THIS_SH} -o posix -c 'trap "echo bad" SIGNOSIG; echo after trap' sh
 
-# this must be last!
-# in posix mode, a function name must be a valid identifier
-# this can't go in posix2.tests, since it causes the shell to exit
-# immediately
+# in posix mode, this is no longer a fatal error
+# a function name does not have to be a valid identifier
 set -o posix
 function !! () { fc -s "$@" ; }
 set +o posix