man: add libudev man-pages (skeletons)

man: add libudev man-pages (skeletons)

This adds man-pages for most of the libudev symbols we export. Similar
symbols are grouped together in a single man-page, with respective links
added. All man-pages contain the full skeleton including NAME, SYNOPSIS,
RETURN VALUE and SEE ALSO. However, most of them still lack the
DESCRIPTION part. This should be copied from the gtkdoc descriptions in
src/libudev/libudev*.[ch]. Any help is welcome! (the whole skeleton is
already done, so it's really just about the prose-part of the man-pages to
be written).

Missing from the man-pages are the following parts:
  - udev_set_log_fn()
  - udev_[gs]et_log_priority()
  - udev_[gs]et_userdata()
  - udev_list_entry_foreach()
  - udev_device_get_seqnum()
  - udev_device_get_usec_since_initialized()
  - udev_util_encode_string()
These are considered legacy, afaik. If not, please feel free to add them
now!

Furthermore, udev-hwdb and udev-queue are not documented at all (for the
same reasons).