Default to +termlib for ncurses for LLVM (#15039)

LLVM is the only package that explicitly sets the "termlib" variant
of ncurses and it specifies +termlib. ncurses defaults to ~termlib;
if a package depends on LLVM and ncurses, there is a concretizer bug
that incorrectly detects a constraint conflict (see #267). Setting
+termlib as the default is a stopgap measure to avoid this conflict.

If other packages were to explicitly request ~termlib in the future,
the same issue would come up again (and could not be resolved by
adjusting the default of "termlib").

Setting +termlib on ncurses moves some symbols into a separate
"libtinfo". Not all packages may be able to detect libtinfo properly
so may require an update; vim, samtools, and libedit have been
updated to use ncurses+termlib (in the case of libedit, the only
necessary action was to add a newer version where the build system
was updated to check libtinfo).

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