ctype, string: depend on extern-inline

This is needed to complete the recent OS X fixes.
Also, fix related documentation as suggested by Eric Blake.
* doc/posix-functions/isalnum.texi, doc/posix-functions/isalpha.texi:
* doc/posix-functions/isascii.texi, doc/posix-functions/iscntrl.texi:
* doc/posix-functions/isdigit.texi, doc/posix-functions/isgraph.texi:
* doc/posix-functions/islower.texi, doc/posix-functions/isprint.texi:
* doc/posix-functions/ispunct.texi, doc/posix-functions/isspace.texi:
* doc/posix-functions/isupper.texi, doc/posix-functions/isxdigit.texi:
* doc/posix-functions/toascii.texi, doc/posix-functions/tolower.texi:
* doc/posix-functions/toupper.texi:
List the 'ctype' gnulib module.
* doc/posix-functions/strcat.texi, doc/posix-functions/strcpy.texi:
* doc/posix-functions/strncpy.texi:
List the 'string' gnulib module.
* modules/memcpy, modules/memmove, modules/memset (Depends-on):
Add string.
* modules/ctype, modules/string (Depends-on): Add extern-inline.