Fix doc quoting problems with Texinfo 5

Without this change, in the info file output, Texinfo 5 quotes code
in text with undirected single quotes 'like this' and generates
code examples that with many PDF readers cannot be cut out of PDFs
and pasted into code.
* manual/libc.texinfo: Configure the libc manual like the GNU
Emacs manual, by using @documentencoding and setting
txicodequoteundirected and txicodequotebacktick.  This way,
Texinfo 5 quotes code in text with directed single quotes ‘like
this’ and produces examples that can be cut out of PDFs.  This
change causes Texinfo 5 to generate info files that contain UTF-8
characters in the set {'‘', '’', '“', '”', 'Ä', 'ä', 'ö', '−',
'–', '—', '©', '⇒', '•', '…'}, which is OK nowadays.