Compilation warnings shushing

With this patch, in clang 3.4.1 we get no compilation complaints if
Local/Makefile contains:

    CC=clang
    CFLAGS+=-Wno-dangling-else -Wno-logical-op-parentheses

* In hash.c, for the OpenSSL case, use assert() to guard the paths which
  can't happen, instead of just assuming that the calling code never has
  a mistake
* Fix some signed/unsigned issues
* Be explicit about some ignored return values
* Some parens around bit-twiddling
* Use our os_getcwd with its extra guards in one place where getcwd was
  called
* FreeBSD: use system iconv, safely, always

(cherry picked from commit 845a3ced80964f562872aba841099adbc8933b40)
Signed-off-by: Phil Pennock <pdp@exim.org>