os-posix.c, softmmu/vl.c: move os_parse_cmd_args() into qemu_init()

This will stop linking softmmu-specific os_parse_cmd_args() into every
qemu executable which happens to use other functions from os-posix.c,
such as os_set_line_buffering() or os_setup_signal_handling().

Also, since there's no win32-specific options, *all* option parsing is
now done in softmmu/vl.c:qemu_init(), which is easier to read without
extra indirection, - all options are in the single function now.

This effectively reverts commit 59a5264b.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-ID: <20230901101302.3618955-5-mjt@tls.msk.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>