numeric backup extensions.
Include "backupfile.h" first.
Include errno.h, stdbool.h, limits.h, unistd.h, xalloc.h.
(CLOSEDIR, INT_STRLEN_BOUND): Remove.
(pathconf) [! (HAVE_PATHCONF && defined _PC_NAME_MAX)]: New macro.
(_POSIX_NAME_MAX) [!defined _POSIX_NAME_MAX]: New macro.
(NAME_MAX_MAXIMUM): New macro. Unlike the old addext.c, we
also look at _XOPEN_NAME_MAX, for better performance on modern
hosts that support only file names of length 255 or more.
(ISDIGIT): unsigned -> unsigned int
(max_backup_version, version_number): Remove.
(check_extension): New function. Similar to the old addext, but
static, assumes that the extension has already been added,
and a bit more careful on DOS hosts.
(numbered_backup): New function. It does what max_backup_version
and version_number used to do, but it doesn't use integer arithmetic
to calculate extensions so it doesn't overflow.
(find_backup_file_name): Rewrite to use these new functions.
This has a new optimization: we needn't call pathconf if the
new numbered backup name has the same length as the old.
Also, use xmalloc rather than malloc, so that the caller
needn't worry about memory exhaustion.
Include <dirname.h> after size_t is defined.
(find_backup_file_name, max_backup_version): Use base_len instead of
rolling it ourselves.
Handle the case of "" and (on DOS) "C:" correctly.
(backup_args): Reorder enum members.
(backup_types): Likewise.
(get_version): Take an additional parameter, `context'.
(xget_version): Like get_version, but if the `version' argument is NULL,
use the value of the envvar VERSION_CONTROL.
(base_name): Declare.
Remove global declaration of backup_type.
(simple_backup_suffix): Default to `~', not `.orig'.
Use PARAMS, not __BACKUPFILE_P.
(find_backup_file_name): Add parameter, backup_type.
