libmisc/yesno.c: Use getline(3) and rpmatch(3)

libmisc/yesno.c: Use getline(3) and rpmatch(3)

getline(3) is much more readable than manually looping.  It has some
overhead due to the allocation of a buffer, but that shouldn't be a
problem here.  If that was a problem, we could reuse the buffer (thus
making the function non-reentrant), but I don't think that's worth the
extra complexity.

Using rpmatch(3) instead of a simple y/n test provides i18n to the
response checking.  We have a fall-back minimalistic implementation for
systems that lack this function (e.g., musl libc).

While we're at it, apply some other minor improvements to this file:

-  Remove comment saying which files use this function.  That's likely
   to get outdated.  And anyway, it's just a grep(1) away, so it doesn't
   really add any value.

-  Remove unnecessary casts to (void) that were used to verbosely ignore
   errors from stdio calls.  They add clutter without really adding much
   value to the code (or I don't see it).

-  Remove comments from the function body.  They make the function less
   readable.  Instead, centralize the description of the function into a
   man-page-like comment before the function definition.  This keeps the
   function body short and sweet.

-  Add '#include <stdbool.h>', which was missing.

-  Minor whitespace style changes (it doesn't hurt the diff at this
   point, since most of the affected lines were already touched by other
   changes, so I applied my preferred style :).

Acked-by: Samanta Navarro <ferivoz@riseup.net>
Cc: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>