harden alias against foo; bar

If the first word of the alias body ends with a semicolon we need to
strip that character, and otherwise escape the extracted command, to
ensure the subsequent function definition is valid.

Fixes #3860

tests/alias.in

@@ -0,0 +1,7 @@
+# Avoid regressions of issue #3860 wherein the first word of the alias ends
+# with a semicolon.
+function foo
+    echo ran foo
+end
+alias my_alias "foo; and echo foo ran"
+my_alias

tests/alias.out

@@ -0,0 +1,2 @@
+ran foo
+foo ran