MFC r302517:

Fix a copy/paste bug introduced during X86_64 Linuxulator work.
FreeBSD support NX bit on X86_64 processors out of the box, for i386 emulation
use READ_IMPLIES_EXEC flag, introduced in r302515.

While here move common part of mmap() and mprotect() code to the files in compat/linux
to reduce code dupcliation between Linuxulator's.

MFC r302518, r302626:

Add linux_mmap.c to the appropriate conf/files.

MFC r294620:

Fix a typo.

MFC r294621:

Remove obsolete comment.

MFC r283487:

Reduce duplication between MD Linux code by moving msg related
struct definitions out into the compat/linux/linux_socket.h

MFC r283474:

Rework signal code to allow using it by other modules, like linprocfs:

1. Linux sigset always 64 bit on all platforms. In order to move Linux
sigset code to the linux_common module define it as 64 bit int. Move
Linux sigset manipulation routines to the MI path.

2. Move Linux signal number definitions to the MI path. In general, they
are the same on all platforms except for a few signals.

3. Map Linux RT signals to the FreeBSD RT signals and hide signal conversion
tables to avoid conversion errors.

4. Emulate Linux SIGPWR signal via FreeBSD SIGRTMIN signal which is outside
of allowed on Linux signal numbers.

PR: 197216

MFC r283471:

According to Linux man sigaltstack(3) shall return EINVAL if the ss
argument is not a null pointer, and the ss_flags member pointed to by ss
contains flags other than SS_DISABLE. However, in fact, Linux also
allows SS_ONSTACK flag which is simply ignored.

For buggy apps (at least mono) ignore other than SS_DISABLE
flags as a Linux do.

While here move MI part of sigaltstack code to the appropriate place.

MFC r283443:

Put the correct value for the abi_nfdbits parameter of kern_select() for
all supported Linuxulators.

MFC r283437:

To avoid code duplication move open/fcntl definitions to the MI
header file.

MFC r283431:

Add AT_RANDOM and AT_EXECFN auxiliary vector entries which are used by
glibc. At list since glibc version 2.16 using AT_RANDOM is mandatory.