manuals: Fix errors in .2 pages

These were reported by mandoc -T lint ... as errors.

fhlink.2, fhreadlink.2: remove unneeded block closing.

getfh.2, procctl.2: add necessary block closing.

ptrace.2: -width only takes one argument.

swapon.2: <sys/vmparam.h> and <vm/swap_pager.h> weren't being displayed,

because .It is for a list item whereas .In is for included files.
Also, we want a blank line between <sys/ > headers and the other
one.

Signed-off-by: Graham Percival <gperciva@tarsnap.com>
PR: 281597
Reviewed by: mhorne
Sponsored by: Tarsnap Backup Inc.