bufferevent_pair: don't call downcast(NULL)
Add -Qunused-arguments for clang on macos

The clang compiler provided with macosx emits warnings like:

  CC       bufferevent.lo
clang: warning: argument unused during compilation: '-I .'
clang: warning: argument unused during compilation: '-I ./compat'
clang: warning: argument unused during compilation: '-I ./include'
clang: warning: argument unused during compilation: '-I ./include'

for each file being compiled. This generates a lot of noise during
compilation making it hard to see "real" errors. This patch mute
those warnings.
Fix a crash in evdns related to shutting down evdns

Patch from YASUOKA Masahiko; fix for libevent github issue #113.
Fix a crash in evdns related to shutting down evdns

(Improved version to deal correctly with probe requests)

Patch from YASUOKA Masahiko; fix for libevent github issue #113.
Add a few files created by "make verify" to .gitignore.
Use FindClose for handle from FindFirstFile in http-server.c

Noted by Miles Chan as issue #145
Consistently check for failure from evbuffer_pullup()

Closes issue #148.
regress_buffer: fix 'memcmp' compare size
[Bugfix] fix bufferevent setwatermark suspend_read