FreeBSD memory (leak) profiling support

Hi Jason,

I've followed the existing approach to use `procfs` to retrieve the virtual memory map of the process being profiled. It assumes it is mounted at `/proc`. That makes for a very simple change in `prof.c` and an additional regex match option in `bin/pprof` for the FreeBSD memory map format.

I've also added an additional lookup for a debug version because `nm` and `addr2line` were not using the same version, which caused havoc on the symbol lookup.

Thanks & cheerio, Harry.