This patch attempts to fix (or aid in debugging) issue #350.

sshuttle didn't explicitly search /sbin and /usr/sbin and they may be
missing in the user's PATH. If PATH is missing, these folders wouldn't
be searched either. There was also a program_exists function which is
redundant to which(). This consolidates everything into the helpers.py
file.

This patch introduces get_path() to return PATH + some extra hardcoded
paths. A new get_env() function can be called to create a consistent
environment when calling external programs. The new which() wrapper
function also ensures we use the same set of paths.

If -vv is supplied, messages clearly indicate the programs we are
looking for, if they are found, and where we looked if we failed to
find them.

I haven't tested the changes to ipfw or pf.