Merge tag 'bsd-user-preen-2022q2-pull-request' of ssh://github.com/qemu-bsd-user...

bsd-user upstreaming: read, write and exit

This series of patches continues the effort to get system calls working
upstream. This series was cleaved off a prior series to give me time to rework
based on the feedback from the first time I posted these. read, write and exit
are implemented, along with a few helper functions and tracing.

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# gpg: Signature made Sat 11 Jun 2022 06:54:51 AM PDT
# gpg:                using RSA key 2035F894B00AA3CF7CCDE1B76C1CD1287DB01100
# gpg: Good signature from "Warner Losh <wlosh@netflix.com>" [unknown]
# gpg:                 aka "Warner Losh <imp@bsdimp.com>" [unknown]
# gpg:                 aka "Warner Losh <imp@freebsd.org>" [unknown]
# gpg:                 aka "Warner Losh <imp@village.org>" [unknown]
# gpg:                 aka "Warner Losh <wlosh@bsdimp.com>" [unknown]
# gpg: WARNING: This key is not certified with a trusted signature!
# gpg:          There is no indication that the signature belongs to the owner.
# Primary key fingerprint: 2035 F894 B00A A3CF 7CCD  E1B7 6C1C D128 7DB0 1100

* tag 'bsd-user-preen-2022q2-pull-request' of ssh://github.com/qemu-bsd-user/qemu-bsd-user:
  bsd-user/freebsd/os-syscall.c: Implement exit
  bsd-user/bsd-file.h: Meat of the write system calls
  bsd-user/bsd-file.h: Add implementations for read, pread, readv and preadv
  bsd-user/freebsd/os-syscall.c: Tracing and error boilerplate
  bsd-user/freebsd/os-syscall.c: unlock_iovec
  bsd-user/freebsd/os-syscall.c: lock_iovec

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>