Use proper mode_t types and macros.
Don't assume the traditional Unix values for mode bits.
modechange.h now includes sys/types.h.
Include xstrtol.h.
(isodigit, oatoi): Remove.
(S_ISUID, S_ISGID, S_ISVTX, S_IRUSR, S_IWUSR, S_IXUSR, S_IRGRP,
S_IWGRP, S_IXGRP, S_IROTH, S_IWOTH, S_IXOTH, S_IRWXU, S_IRWXG,
S_IRWXO): Define if not defined.
(CHMOD_MODE_BITS): New macro.
(mode_compile): Convert from octal with xstrtoul, not our own routine.
(mode_append_entry): Likewise.
(mode_compile): When none of [ugoa] is specified in an `=OP' change
mode request, insert a `=0' entry into the linked list so that all
bits are cleared first. Use the new functions.
