Adding a callstack whitelist via MOCKEAGAIN_WL

[mtourne]

(from the doc added in README.markdown)

This environment variable can be set to a comma separated list of function names.

If it is set, mockeagain will call the original libc function if one of whitelisted function is found in the call stack.

For example:

MOCKEAGAIN_WL='ngx_ssl_handshake, ngx_ssl_create' MOCKEAGAIN=rw MOCKEAGAIN_VERBOSE=1 LD_PRELOAD=/path/to/mockeagain.so /path/to/nginx ...

Will let you use mockeagain for HTTP (or SPDY) connections, since the first packets for certificate exchange are always expected to be transmitted in a single TCP packet (versus 1 byte at a time).