Remove a ton of duplicate and obsolete code from configure.

- Always exit with EXIT_FAILURE on error.
- Always replace the old config.h with out new one.
- Clean up the ./configure --help message slightly.
- Install dhparams.pem into the conf directory on GnuTLS as well as
  OpenSSL.
- Only show the users InspIRCd Git revision when building from Git.
- Remove a check for MinGW (we have native Windows builds).
- Remove a check for FreeBSD 4 (released in 2000).
- Remove a ton of pointless tab-indented comments.
- Remove a ton of duplicate or unused variables.
- Remove the check for stdint.h (always available on TR1 compilers).
- Remove the $writeheader argument from writefiles (always 1).
- Remove the following %config options:
    - DESTINATION (not used, always set to BASE).
    - EXECUTABLE (always set to inspircd).
    - OSNAME (duplicate of SYSTEM).
- Remove support for checking whether -extra modules are up to
  date (if the user is installing they won't have any -extra
  modules installed).
- Remove SSL generation messages (genssl provides these).
- Replace inappropriate uses of printf with print.
- Replace is_dir with -d from the Perl stdlib.
- Replace resolve_directory with rel2abs from the Perl stdlib.
- Rewrite getrevision to be more compact, cache the revision and
  return 'release' on non-Git builds.
- Use consistent messages for all tests.