Fix #104, #54: issues with overlong shebang in deep directories.

This does several things:

- Add `sbang`: a script to run scripts with long shebang lines.
  - Documentation for `sbang` is in `bin/sbang`.

- Add an `sbang` hook that filters the `bin` directory after install
  and modifies any scripts wtih shebangs that are too long to use
  `sbang` instead.

- `sbang` is at the top level, so it should be runnable (not much we
  can do if spack itself is too deep for shebang)
  - `sbang`, when used as the interpreter, runs the *second* shebang
    line it finds in a script.
  - shoud fix issues with too long shebang paths.

bin/sbang

@@ -0,0 +1,84 @@
+#!/bin/bash
+#
+# `sbang`: Run scripts with long shebang lines.
+#
+# Many operating systems limit the length of shebang lines, making it
+# hard to use interpreters that are deep in the directory hierarchy.
+# `sbang` can run such scripts, either as a shebang interpreter, or
+# directly on the command line.
+#
+# Usage
+# -----------------------------
+# Suppose you have a script, long-shebang.sh, like this:
+#
+#     1    #!/very/long/path/to/some/interpreter
+#     2
+#     3    echo "success!"
+#
+# Invoking this script will result in an error on some OS's.  On
+# Linux, you get this:
+#
+#     $ ./long-shebang.sh
+#     -bash: ./long: /very/long/path/to/some/interp: bad interpreter:
+#            No such file or directory
+#
+# On Mac OS X, the system simply assumes the interpreter is the shell
+# and tries to run with it, which is likely not what you want.
+#
+#
+# `sbang` on the command line
+# -----------------------------
+# You can use `sbang` in two ways.  The first is to use it directly,
+# from the command line, like this:
+#
+#     $ sbang ./long-shebang.sh
+#     success!
+#
+#
+# `sbang` as the interpreter
+# -----------------------------
+# You can also use `sbang` *as* the interpreter for your script. Put
+# `#!/bin/bash /path/to/sbang` on line 1, and move the original
+# shebang to line 2 of the script:
+#
+#     1    #!/bin/bash /path/to/sbang
+#     2    #!/long/path/to/real/interpreter with arguments
+#     3
+#     4    echo "success!"
+#
+#     $ ./long-shebang.sh
+#     success!
+#
+# On Linux, you could shorten line 1 to `#!/path/to/sbang`, but other
+# operating systems like Mac OS X require the interpreter to be a
+# binary, so it's best to use `sbang` as a `bash` argument.
+# Obviously, for this to work, `sbang` needs to have a short enough
+# path that *it* will run without hitting OS limits.
+#
+#
+# How it works
+# -----------------------------
+# `sbang` is a very simple bash script. It looks at the first two
+# lines of a script argument and runs the last line starting with
+# `#!`, with the script as an argument. It also forwards arguments.
+#
+
+# First argument is the script we want to actually run.
+script="$1"
+
+# Search the first two lines of script for interpreters.
+lines=0
+while read line && ((lines < 2)) ; do
+    if [[ "$line" = '#!'* ]]; then
+        interpreter="${line#\#!}"
+    fi
+    lines=$((lines+1))
+done < "$script"
+
+# Invoke any interpreter found, or raise an error if none was found.
+if [ -n "$interpreter" ]; then
+    exec $interpreter "$@"
+else
+    echo "error: sbang found no interpreter in $script"
+    exit 1
+fi