Description:
------------
Like nginx (nginx -p) or apache (httpd -d), FPM should have a command line 
option to set a custom prefix on which all relative file path can depend on.

"php-fpm -p /var/www" will look for /var/www/etc/php-fpm.conf.
pid=logs/php-fpm.pid ; will expand to /var/www/logs/php-fpm.pid
...

Moreover it could be great to have a custom prefix per pool also.

php-fpm -p /var/www

; /var/www/etc/php-fpm.conf
pid=logs/php-fpm.pid
[www1]
prefix = www1 ; expand to /var/www/www1
include = fpm.default.conf
[www2]
prefix = www2 ; expand to /var/www/www2
include = fpm.default.conf

; fpm.default.conf
listen = logs/php-fpm.sock ; expand to /var/www/www{1,2}/logs/php-fpm.sock
pm = static
pm.max_children = 50
slowlog = logs/php-fpm.slowlog ; expand to /var/www/www{1,2}/logs/php-
fpm.slowlog
chroot = $prefix ; expand to /var/www/www{1,2}
chdir = /docs
php_value[error_log] = /logs/php.error.log
...