pg_upgrade: Move live_check variable to user_opts.

pg_upgrade: Move live_check variable to user_opts.

At the moment, pg_upgrade stores whether it is doing a "live check"
(i.e., the user specified --check and the old server is still
running) in a local variable scoped to main().  This live_check
variable is passed to several functions.  To further complicate
matters, a few call sites provide a hard-coded "false" as the
live_check argument.  Specifically, this is done when calling these
functions for the new cluster, for which any live-check-only paths
won't apply.

This commit moves the live_check variable to the global user_opts
variable, which stores information about the options the user
specified on the command line.  This allows us to remove the
live_check parameter from several functions.  For the functions
with callers that provide a hard-coded "false" as the live_check
argument (e.g., get_control_data()), we verify the given cluster is
the old cluster before taking any live-check-only paths.

This small refactoring effort helps simplify some proposed changes
that would parallelize many of pg_upgrade's once-in-each-database
tasks using libpq's asynchronous APIs.  By removing the live_check
parameter, we can more easily convert the functions to callbacks
for the new parallel system.

Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/20240516211638.GA1688936%40nathanxps13