v0.18.0: Beta release #3

This is a beta release focused on improving user experience, which also introduces some minor new features.

Thanks to current sponsors: Paweł Zmarzły, Wesley Moore, brazeon, q66

Changes:

• For user instances, if the XDG_CONFIG_HOME environment is set, dinit now looks for services in $XDG_CONFIG_HOME/dinit.d/ in addition to $HOME/.config/dinit.d (if they are different paths).
  Implemented by Oliver Amann.
  Note that Dinit does not, and most likely never will, fully support the (flawed) XDG Base Directory Specification.

New features:

• dinitcheck now supports an "online" mode of operation (--online, -n) in which it will query the service directories and service manager environment from a running dinit instance.
• In dinit-monitor: support reporting initial service status, and using alternative (user-supplied) wording for status texts. Thanks to James Knippes (helican).
• For service descriptions, a new form of variable expansion is available. When written as $/NAME (or $/{NAME}) the variable value undergoes word splitting and whitespace collapse. This is convenient for allowing optional command line arguments to be specified via the environment, for example.

Fixes:

• Fix a latent control protocol processing issue (does not affect dinitctl).
• Various minor improvements to error messages, designed to provide a better end-user experience.
• Fixes to minor build issues in Meson build (Mobin Aydinfar).