(base_64_digits): New constant.

(base_64_digits): New constant.
(base_8_digits): New macro.
(MAX_VAL_WITH_DIGITS): New macro.
(to_base): Renamed from to_oct.  Support base 64 too.
New parameters specifying bits per digit and digits.
Remove `type' parameter.  The result is now undefined if it doesn't fit;
it's the caller's responsibility to check this.
(to_chars): Other half of old to_oct, for 64-bit support.
Mostly a new function.
(GID_NOBODY, UID_NOBODY): Don't define if the headers don't.
(gid_substitute, uid_substitute): Look up names dynamically if
GID_NOBODY and UID_NOBODY aren't defined; use -2 if all else fails.
(mode_to_chars): Renamed from mode_to_oct.
Support negative values in all the _to_chars functions.
(start_header): Use FILESYSTEM_PREFIX_LEN instead of MSDOS ifdef.
Abort if archive format is DEFAULT_FORMAT when it shouldn't be.
(dump_file): Inspect entire pathname, not just new file name
component, when deciding whether to exclude it.