Set maximum name length in Nix

Previously we allowed any length of name for Nix derivations. This is
bad because different file systems have different max lengths. To make
things predictable, I have picked a max. This was done by trying to
build this derivation:

  derivation {
    name = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
    builder = "/no-such-path";
    system = "x86_64-linux";
  }

Take off one a and it will not lead to file name too long. That ends
up being 212 a’s. An even smaller max could be picked if we want to
support more file systems.

Working backwards, this is why:

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa-${name}.drv.chroot

> 255 - 32 - 1 - 4 - 7 = 211

src/libstore/store-api.cc

@@ -97,6 +97,10 @@ void checkStoreName(const string & name)
        reasons (e.g., "." and ".."). */
     if (string(name, 0, 1) == ".")
         throw Error(baseError % "it is illegal to start the name with a period");
+    /* Disallow names longer than 211 characters. ext4’s max is 256,
+       but we need extra space for the hash and .chroot extensions. */
+    if (name.length() > 211)
+        throw Error(baseError % "name must be less than 212 characters");
     for (auto & i : name)
         if (!((i >= 'A' && i <= 'Z') ||
               (i >= 'a' && i <= 'z') ||