This adds "switch" true/false flag in "terminal" section of
configuration file.
Flag controls whether terminal under control should be switched to
when greetd starts. If "switch" set to true, greetd behaves as it
did before, on start vt_setactivate will be called. If "switch" set
to false, and terminal under control by greetd is not currently
active VT, greetd will wait for terminal to become active with
vt_waitactive, which translates to VT_WAITACTIVE ioctl call.
* greetd/src/config/mod.rs: add "switch" flag
* greetd/src/server.rs: add using "switch" flag and waiting for active
* greetd/src/terminal/mod.rs: add vt_waitactive method
* man/greetd-5.scd: mention "switch" configuration option
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