module: neaten __find_symbol, rename to find_symbol

__find_symbol() has grown over time: there are now 5 different arrays
of symbols it traverses.  It also shouldn't print out a warning on
some calls (ie. verify_symbol which simply checks for name clashes,
and __symbol_put which checks for bugs).

1) Rename to find_symbol: no need for underscores.
2) Use bool and add "warn" parameter to suppress warnings.
3) Make table-driven rather than open coded.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>