Add support for serving directly from filesystem

You can now pass `-D directory` to redbean which will serve assets from
the local filesystem. This is useful for development since it allows us
to skip needing to shut down the server and run InfoZIP when testing an
iteration of a lua server page script.

See #97

net/http/isacceptablehttprequestpath.c

@@ -0,0 +1,75 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2021 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/str/thompike.h"
+#include "net/http/http.h"
+
+/**
+ * Returns true if request path seems legit.
+ *
+ * 1. Request path must start with '/'.
+ * 2. The substring "//" is disallowed.
+ * 3. We won't serve hidden files (segment starts with '.').
+ * 4. We won't serve paths with segments equal to "." or "..".
+ *
+ * It is assumed that the URI parser already took care of percent
+ * escape decoding as well as ISO-8859-1 decoding. The input needs
+ * to be a UTF-8 string.
+ */
+bool IsAcceptableHttpRequestPath(const char *data, size_t size) {
+  bool t;
+  size_t i;
+  unsigned n;
+  wint_t x, y, a, b;
+  const char *p, *e;
+  if (!size || *data != '/') return false;
+  t = 0;
+  p = data;
+  e = p + size;
+  while (p < e) {
+    x = *p++ & 0xff;
+    if (x >= 0300) {
+      a = ThomPikeByte(x);
+      n = ThomPikeLen(x) - 1;
+      if (p + n <= e) {
+        for (i = 0;;) {
+          b = p[i] & 0xff;
+          if (!ThomPikeCont(b)) break;
+          a = ThomPikeMerge(a, b);
+          if (++i == n) {
+            x = a;
+            p += i;
+            break;
+          }
+        }
+      }
+    }
+    if (x == '\\') {
+      x = '/';
+    }
+    if (!t) {
+      t = true;
+    } else {
+      if ((x == '/' || x == '.') && y == '/') {
+        return false;
+      }
+    }
+    y = x;
+  }
+  return true;
+}