cmsg(3): Clarify guarantees about CMSG_SPACE/LEN constancy.

Add some line breaks while here to break up walls of text so it's
easier to compare the two paragraphs and see how CMSG_SPACE is meant
for struct msghdr::msg_controllen and CMSG_LEN is meant for struct
cmsghdr::cmsg_len.

PR lib/59054: cmsg(3) uses malloc(3) unnecessarily

cmsg(3): Handle as many descriptors as the sender sends in example.

Due to a quirk of alignment, sizing the control buffer for
CMSG_SPACE(sizeof(int)) does not guarantee at most one file
descriptor fits in the buffer -- so if there is a chance the sender
may send more than one descriptor, the receiver has to loop over all
of them to avoid leaks.

Prompted by:

PR lib/59054: cmsg(3) uses malloc(3) unnecessarily

cmsg(3): Don't use malloc in example.

Since CMSG_SPACE (and CMSG_LEN) is required to expand to an integer
constant expression by POSIX, and it does in NetBSD as of 2012, there
is no reason to use malloc here -- we can just allocate the buffer on
the stack without tripping over variable-length arrays.

PR lib/59054: cmsg(3) uses malloc(3) unnecessarily

branches: 1.3.28;
correct CMSG_DATA documentation

branches: 1.2.2;
and adjust it to our liking.

steal the OpenBSD man page.