Made the names of the function arguments of the declaration and the definition...

Made the names of the function arguments of the declaration and the definition of mg_get_var(..) the same.

Changed the error codes to have a consistent way of reporting errors. -2 is returned if there is a problem with the size of the destination buffer. -1 is returned if the variable is not found.

Updated the documentation of mg_get_var(..) in mongoose.h and removed the redundant comments from mongoose.c.