Fix C generation for typedef'd enums

This was broken in the gen.py refactor, so I've added a test.
Add non-functional longdouble type

This is to prevent having to blacklist strtold.
Better error when calling function incorrectly

Slightly better.
Add check to bin/gen.py for NACL_SDK_ROOT

This gives a nicer error if it is not set.
Fix comment parsing regexp in bin/gen.py

I've separated into a C- and C++-style comment RE.
Allow remapping function symbols.

You can now pass "-r foo=bar" to gen.py and it will remap the function
named "foo" to "bar", when being called from JavaScript. This allows for
overloading functions such as "set", "get", etc.