string_bytes: don't use named imports in header

It imports the definition into every source file that includes
string_bytes.h, as evidenced by the build suddenly breaking left
and right because of missing Handle/Local/String/Value imports.
src: remove unused import in src/node_os.cc
src: const-ify variables in src/node_crypto*

No functional changes, just some code tightening. Clean up some style
inconsistencies while we are here.
crypto: simplify DH modp group name matcher

* Use ARRAY_SIZE() rather than scanning until we hit a NULL entry.
* Fix `-fsigned-char -Wnarrowing` compiler warnings. Harmless but
  numerous and annoying.
* Static-ify the modp_group and mod_groups arrays.
* Const-ify the modp_groups array.