libc: Roll {l,ll,imax}abs(3) manpages into just abs(3)

No need to have 4 separate manpages for these functions. Use opportunity
to change parameter names in the source from j -> i to reflect the name
used in POSIX. (The ISO C standard uses j but i is a better name
anyway.)

Reviewed by: des, rpokala
Approved by: rpokala
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D55361