Allow builtin source to read from non-regular files

Commit a91bf6d88 (builtin.c: builtin_source now checks that its argument is
a file., 2005-12-16) fixed an infinite loop for commands like "source /"
where the argument is a directory.

It did so by erroring out early unless the filename argument is a regular file.
This is too restrictive; it disallows reading from special files like /dev/null
and fifos.
Today we get a sensible error without this check, so remove it.

tests/checks/source.fish

@@ -0,0 +1,11 @@
+#RUN: %fish %s
+
+source /dev/null
+echo $status
+# CHECK: 0
+
+source /
+echo $status
+# CHECKERR: error: Unable to read input file: Is a directory
+# CHECKERR: source: Error while reading file '/'
+# CHECK: 1