mobile-shell: teach mosh where the Cellar is for non-terminal use

When mosh is launched outside a terminal, it's likely the PATH may not include the brew prefix, which means that when mosh (the perl script) calls mosh-client (the binary protocol handler) it won't be found.
 
This change rewrites line 42 & 43 in `mosh` to use the full brewed PATH inside the cellar. 

In practice, you can now add for example in iTerm2 or from launch tools like Alfred, `mosh my.serv.er` and it will Just Work.