Use current directory for .pm2 when no $HOME defined

[gierschv]

$ pm2 -v
0.12.1
$ unset HOME
$ pm2 -v    

path.js:313
        throw new TypeError('Arguments to path.resolve must be strings');
              ^
TypeError: Arguments to path.resolve must be strings
    at Object.exports.resolve (path.js:313:15)
    at Object.<anonymous> (/home/vgiersch/bin/node-v0.10.26-linux-x64/lib/node_modules/pm2/constants.js:15:21)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Module.require (module.js:364:17)
    at require (module.js:380:17)
    at Object.<anonymous> (/home/vgiersch/bin/node-v0.10.26-linux-x64/lib/node_modules/pm2/lib/Satan.js:18:14)
    at Module._compile (module.js:456:26)

[soyuka]

Nice idea, and could be a fix for #789

[gierschv]

@soyuka For #789, it depends what is the cwd. Maybe we should use project directory instead of current working directory when no $HOME, or just throw a clean error message. Anyway, a better option is to set $PM2_HOME.

[Tjatse]

To be honest, I don't think this is a good idea, path.resolve('') returns the CWD, that means all the PM2 confs (like logs, pids, pm2*) will be putted in to CWD, thats inadvisable. e.g.:
BASH

$ cd /var/www/my-codes
$ unset HOME
$ unset PM2_HOME
$ pm2 start test.js

You'll find there is such a cram in the /var/www/my-codes directory.
DIRS

/var/www/my-codes/.pm2/logs
/var/www/my-codes/.pm2/pids
/var/www/my-codes/.pm2/pm2.log
...

How about we have another project under /var/www/my-codes2, after we ran the BASH
Now the DIRS

/var/www/my-codes2/.pm2/logs
/var/www/my-codes2/.pm2/pids
/var/www/my-codes2/.pm2/pm2.log
...

Yap, the logs do not tangle up anymore.

Taking ~/.pm2 for the ROOT-DIR of PM2 is a good idea but not best, 'Cause we always facing root/nobody user problems (or su/sudo to start PM2 then downgrade permission to nobody), actually I think /opt/pm2 should be ROOT-DIR by default:

/opt is for "the installation of add-on application software packages", it is a directory where to install unbundled packages, each in its own subdirectory.