Enforce exclusive output options

Currently, any output option (e.g., --version or --libs) will be set as
valid and what's output is at the mercy of the order of the output
handling code in main(). However, most combinations of output would make
no sense to be used together. For example, mixing --modversion and
--print-provides provides no way to differentiate between the output
from the options. Further, mixing --variable and --cflags currently
causes an error because there's no space separating the option outputs.

Instead, keep track of when an output option has been set and ignore
subsequent output options. There are currently two exceptions:

1. Any combination of --cflags* and --libs* are allowed.

2. Both --print-requires and --print-requires-private can be used
   together as the user may just not care which is private.

Freedesktop #54391 (https://bugs.freedesktop.org/show_bug.cgi?id=54391)

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