mamake: reduce backward compat stubs (re: 1aecc1b)

Archaeological research on the Mamfiles in the ast-open-history
repo shows that the assignment-argument parsing code in the
'silent' and 'ignore' scripts was never used, because no
assignment-argument (e.g.: silent var=value somecommand) was ever
passed. So we don't need to try too hard to be backward compatible
with this.

src/cmd/INIT/mamake.c: run():
- Reduce the 'silent' stub to an empty alias. Any theoretical
  assignment-arguments will be true shell assignments (e.g.:
  'var=value somecommand') instead; the effect should be the same.
- Reduce the 'ignore' prefix to an 'env' command invocation plus
  ':' to ensure a zero exit status; 'env' will process any
  theoretical assignment-arguments. This is not safe for field
  splitting, but then again, this whole system never was.
- Since we're not using them any more, don't bother setting them
  at all in MAMAKE_STRICT mode.

src/cmd/INIT/README-mamake.md

@@ -33,6 +33,8 @@ ksh 93u+m made a few changes to `mamake` that make it easier to maintain Mamfile
 * Indentation and word separators may use any whitespace (e.g. tabs), not only spaces.
 * Unrecognized commands and rule attributes throw an error instead of being silently ignored.
 * Fixed some crashing bugs and memory leaks.
+* The `silent` and `ignore` command prefixes for shell actions are processed internally
+  for backward compatibility, unless the `MAMAKE_STRICT` variable is set.
 * The `prev` command may now be used without a prior `make`...`done` to declare a simple
   prerequisite, provided the `MAMAKE_STRICT` variable is set.
 * The `notrace` attribute was added to disable xtrace for a rule's shell action.

src/cmd/INIT/mamake.c

@@ -1276,24 +1276,14 @@ run(Rule_t* r, char* s)
 		x = state.exec;
 	if (x)
 	{
+		/* stubs for backward compat */
+		if(!strict())
+			append(buf,
+				"alias silent=\n"
+				"ignore() { env \"$@\" || :; }\n"
+			);
+		/* find commands in the current working directory first */
 		append(buf,
-			/* stub for nmake's silent prefix (for backward compat) */
-			"silent()\n"
-			"(\n"
-				"while	test \"$#\" -gt 0\n"
-				"do	case $1 in\n"
-					"*=*)	export \"$1\"; shift;;\n"
-					"*)	break;;\n"
-					"esac\n"
-				"done\n"
-				"\"$@\"\n"
-			")\n"
-			/* stub for nmake's ignore prefix (for backward compat) */
-			"ignore()\n"
-			"{\n"
-				"silent \"$@\" || :\n"  /* always return status 0 */
-			"}\n"
-			/* find commands in the current working directory first */
 			"case $PATH in\n"
 			".:*)	;;\n"
 			"*)	PATH=.:$PATH;;\n"