Added more nesc options to the man page

[bradjc]

This is a little tricky as there are two perl scripts involved as well as the actual compiler.

There are a couple flags that I can't tell what they do:

• -fnesc-msg=message
• -fnesc-csts
• -fnesc-genprefix=prefix

Fixes issue #18

[dgay42]

Thanks for doing this! (most of the comments relate to options that are for internal use)

[bradjc]

I updated the commit with your changes and made notes in nesc-main.c where necessary.

[bradjc]

I definitely forgot about that top header options block. Should be better now.