MFC r310088, r310090, r310095

r310088:
Put the undocumented df feature of mounting filesystems from device
nodes
under an ifdef. Leave enabled.

Reviewed by: cem
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D8513

r310090:
Mount filesystems without executable permissions since they should never
be used.

Reviewed by: cem
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D8513

r310095:
Use nmount(2) rather than the obsolete mount(2).

Reviewed by: cem
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D8513

MFC r287236:

Use exit() instead of return in main(). The difference in practice
is subtle: C standard requires the language runtime to make return
of int from main() behave like calling exit(), and in FreeBSD we do:

exit(main(argc, argv, env))

In lib/csu/${ARCH}/crt1.c, so the real difference is using exit()
explicitly would use an additional stack frame.

Note however, if there is a on stack pointer is the last reference
of an allocated memory block, returning from the function would,
technically, result in a memory leak because we lost the last
reference to the memory block, and calling exit() from C runtime
could potentionally overwrite that stack frame that used to belong
to the main() function.

In practice, this is normally Okay because eventually the kernel
would tear down the whole address space that belongs to the process
in the _exit(2) system call, but the difference could confuse
compilers (which may want to do stack overflow checks) and static
analyzers.

Replacing return with exit() in main() allows compilers/static
analyzers to correctly omit or generate the right warnings when
they do not treat main() specifically. With the current version
of clang on FreeBSD/amd64, use of exit() would result in slightly
smaller code being generated and eliminated a false positive
warning of memory leak.

MFC r287236:

Use exit() instead of return in main(). The difference in practice
is subtle: C standard requires the language runtime to make return
of int from main() behave like calling exit(), and in FreeBSD we do:

exit(main(argc, argv, env))

In lib/csu/${ARCH}/crt1.c, so the real difference is using exit()
explicitly would use an additional stack frame.

Note however, if there is a on stack pointer is the last reference
of an allocated memory block, returning from the function would,
technically, result in a memory leak because we lost the last
reference to the memory block, and calling exit() from C runtime
could potentionally overwrite that stack frame that used to belong
to the main() function.

In practice, this is normally Okay because eventually the kernel
would tear down the whole address space that belongs to the process
in the _exit(2) system call, but the difference could confuse
compilers (which may want to do stack overflow checks) and static
analyzers.

Replacing return with exit() in main() allows compilers/static
analyzers to correctly omit or generate the right warnings when
they do not treat main() specifically. With the current version
of clang on FreeBSD/amd64, use of exit() would result in slightly
smaller code being generated and eliminated a false positive
warning of memory leak.