execve: Support for switching user
php-fpm: Just take the config and ini as arguments
Introduce run-as-user for user switching

By default we compile a wrapper that calls setuid before execing the
program, but the program can specialize running as another user.
multiplex-activations specializes it to run the *wrapped* program as the
new user (so that resources such as sockets are acquired as root) and
execve specializes it to call setuid in the same process.
php-fpm: Don't specify user, that can be overridden externally
Add packages to run a set of programs in parallel or sequentially
Only allow specifying a single initializer

seq and par can be used to combine them

Fixes #20
nginx: Just take a config file directly
Remove unused dirs initializer
Initializers are just pkgs, nothing special
When the target and eval-time system differ, usually 'system' is globally overridden
Missing word
run-as-user -> run-with-settings
Add a convencience service wrapper for running a service with execution settings
Argument renaming
Fix run-with-settings arguments
ints can't be coerced to string
Syntax errors
Fix compilation
Don't send readiness notification before the service is listening