Remove the global variable 'info' and fix associated bugs and style bugs.

This variable was used 4 times in 1 function and all uses were wrong.
The 4 uses were in he test_frame() (show) function, to try to restore
4 colors, 2 unnecessarily and these 2 now broken. This was wrong
because it is the previous colors that must be restored, but the global
holds the original colors. Excessive setting of colors at the end
restored the previous colors correctly in most cases, but I removed
this a couple of revisions ago.

Originally, this variable had 1 correct use, to test for being on a vty
as a side effect of initializing it. This is now down in init(), and
init() also leaves a better-named global with the same contents.

Fix this by reading the current console info into a local variable in
test_frame(), as is done for several other functions. Fix style bugs
in this reading for all callers:

• extra blank lines
• all error messages different. The first one now in init() is not as specific as the old one, but it is after a different specific one for another ioctl and is unlikely to be reached when the first ioctl succeeds. Ones after the first are to repeat the ioctl, so are even more likely to be reached. The correctness of full removal of the old global depends on the error handling for failure to initialize it being unreachable.
• err() instead of warn() for failure in load_font(). This is almost unreachable, and it makes no sense to continue after undoing previous changes with revert().
• unreachable return after err() for failure in dump_screen().

Undo large renaming of local variables from the good name 'info' to the
bad name _info, which was done to protect the buggy global's bad name
from -Wshadow warnings.