modif: lookup xauth in PATH

[chestnykh]

Don't use hardcoded /usr/bin/xauth,
iterate over directories inside PATH instead.

This fixes #6006

[glitsj16]

Thanks for the patch!

[kmk3]

I see now that there already is a function for doing this: find_in_path() in
fs_lib.c:

• firejail/src/firejail/fs_lib.c

  Line 55 in b689b69

  char *find_in_path(const char *program) {

Please try using that instead of adding a new one.

[chestnykh]

I see now that there already is a function for doing this: find_in_path() in fs_lib.c:

* https://github.com/netblue30/firejail/blob/b689b69f6c3b8a8ba633d6300cef6a19972d53dc/src/firejail/fs_lib.c#L55

Please try using that instead of adding a new one.

done

[kmk3]

Looks better now, thanks.

Note: There is still a TOCTOU issue with the checks due to this allowing a
potentially user-writable xauth binary, though fixing the issue properly
requires refactoring elsewhere, so I might try that after this PR.

[netblue30]

Merged, thanks!

[netblue30]

Merged, thanks!