Improve error handling for posix file syscalls

[markj]

Summary

At the moment, both syscalls store the error number in td_retval[0] and
return 0, indicating success. This has two problems:

• td_errno is not set, so errors are not made visible to ktrace and dtrace in the usual way
• negative error numbers such as ERESTART, which are supposed to be interpreted by the kernel, are returned to userland

Thus, set td_errno and use TDP_NERRNO to ensure that it is not set by
the kernel syscall layer, and return negative errors directly.

[kib]

You also need to fix freebsd32_posix_f*() wrappers from sys/compat/freebsd32/freebsd32_misc.c

sys/kern/vfs_syscalls.c

4461

posix_ret_error() might be better. For some reason this function should be non-static anyway, and I expect that more posix functions with this interface would come.

[markj]

Fixups based on feedback.

[markj]

sys/kern/vfs_syscalls.c

4461

Perhaps syscall_posix_ret(), to fit in with the kern_syscalls.c namespace?

[kib]

sys/kern/kern_syscalls.c

234 ↗

(On Diff #13693)

kern_syscalls.c is somewhat unnatural place as well, the file is for stuff related to the syscall registration and syscall module loading. Might be sys_generic.c is better place.

sys/kern/vfs_syscalls.c

4461

Good name.

sys/sys/sysent.h

273 ↗

(On Diff #13693)

sysent.h is unsuitable, I think. syscallsubr.h is the good place IMO, it is the place for syscall-related helpers.

[markj]

sys/sys/sysent.h

273 ↗

(On Diff #13693)

Ok. I had looked at syscallsubr.h (which sounds like the right place), but it's currently only used for the kern_* syscall implementation routines; sysent.h is used for various ad-hoc helpers.

[kib]

sys/sys/sysent.h

273 ↗

(On Diff #13693)

sysent.h is for the ABI vectors related declarations.

[kib]

sys/kern/vfs_syscalls.c

4461

Call it kern_posix_error(), it is both more regular and match the syscallsubr.h namespace conventions.

[markj]

• More fixups

[markj]

This was committed... not sure why that's not happening automatically anymore.