- fix bug where the 'clear' flag was set outside of main loop, so
"dump,uchg,arch" on a file with flags would clear all the bits
(because "dump" sets 'clear'). noted in FreeBSD PR 10071, and fix
obtained from FreeBSD.
- enforce right-most priority on arguments by clearing the appropriate
bit in the opposite set; if a string of "nouchg,uchg" is given,
chflags(8) would clear uchg because it applies the "set" bits before
clearing the "clear" bits. now, "uchg" sets UF_IMMUTABLE in setp
and also resets it in clrp, solving this problem
- constify second arg (the `default' arg)
