build-sys: meson: Allow building only the daemon

This is continuation of daemon/client split on top of !657 (closed) as agreed with @leio

• replace meson option daemon-only with inverse option client
• clean up remaining tests to fall under 'daemon' and 'client'
• rearrange dependency detection to make client/daemon separation more clear
• fix bash completion to not install same file twice when client and daemon are built separately