Allow boostrapping xinstall on Linux

[arichardson]

Summary

Linux does not have st_flags so we have to avoid using it there.

[emaste]

usr.bin/xinstall/xinstall.c

83 ↗

(On Diff #64580)

We're sufficiently confident that UF_SETTABLE will remain undefined on Linux? I guess it's not a big deal if this breaks and needs to be fixed, but probably worth a comment here at least.

[arichardson]

usr.bin/xinstall/xinstall.c

83 ↗

(On Diff #64580)

Since this is the mask of st_flags values, I would assume support it existing implies st_flags also exists.
This was suggested by @imp in D13307.

[imp]

usr.bin/xinstall/xinstall.c

83 ↗

(On Diff #64580)

We are as confident as we can be about Linux...