MFC r364936, r365023

lib: add libnetmap

This changeset introduces the new libnetmap library for writing
netmap applications.
Before libnetmap, applications could either use the kernel API
directly (e.g. NIOCREGIF/NIOCCTRL) or the simple header-only-library
netmap_user.h (e.g. nm_open(), nm_close(), nm_mmap() etc.)

The new library offers more functionalities than netmap_user.h:

• Support for complex netmap options, such as external memory allocators or per-buffer offsets. This opens the way to future extensions.
• More flexibility in the netmap port bind options, such as non-numeric names for pipes, or the ability to specify the netmap allocator that must be used for a given port.
• Automatic tracking of the netmap memory regions in use across the open ports.

At the moment there is no man page, but the libnetmap.h header file
has in-depth documentation.

Reviewed by: hrs
Differential Revision: https://reviews.freebsd.org/D26171