maint: straggler *.[ch] files: convert more `...' to '...'

The preceding commands ignored .[ch] files in lib/ and gl/.
This is what I should have been doing from the start:

  git grep -l '`.*'\' $(g ls-files |grep '\.[ch]$') \
    | xargs perl -pi -e 's/`(.+?'\'')/'\''$1/'

gl/lib/root-dev-ino.c

@@ -1,4 +1,4 @@
-/* root-dev-ino.c -- get the device and inode numbers for `/'.
+/* root-dev-ino.c -- get the device and inode numbers for '/'.
    Copyright (C) 2003, 2005-2006, 2009-2012 Free Software Foundation, Inc.
 
    This program is free software: you can redistribute it and/or modify
@@ -22,7 +22,7 @@
 
 #include <stdlib.h>
 
-/* Call lstat to get the device and inode numbers for `/'.
+/* Call lstat to get the device and inode numbers for '/'.
    Upon failure, return NULL.  Otherwise, set the members of
    *ROOT_D_I accordingly and return ROOT_D_I.  */
 struct dev_ino *

lib/fd-reopen.c

@@ -26,7 +26,7 @@
 #include <unistd.h>
 
 /* Open a file to a particular file descriptor.  This is like standard
-   `open', except it always returns DESIRED_FD if successful.  */
+   'open', except it always returns DESIRED_FD if successful.  */
 
 int
 fd_reopen (int desired_fd, char const *file, int flags, mode_t mode)

lib/strnumcmp-in.h

@@ -34,7 +34,7 @@
    - It's typically faster.
    POSIX says that only '0' through '9' are digits.  Prefer ISDIGIT to
    isdigit unless it's important to use the locale's definition
-   of `digit' even when the host does not conform to POSIX.  */
+   of 'digit' even when the host does not conform to POSIX.  */
 # define ISDIGIT(c) ((unsigned int) (c) - '0' <= 9)