inet.4 tcp.4 udp.4: Replace *CTL_* OID macro names with sysctl names

Older sysctls with constant OID macros were identified with those
in inet.4, tcp.4, and udp.4; newer sysctls with automatic numbering
were identified by sysctl names. No one remembers the OID macros,
or knows what they are; sysctls are always done by name now, usually
via sysctl(8).

Replace the OID macro names with sysctl names so that there is one
uniform identifier type; sysctl names were previously in parens.
Make the formatting a little more consistent in this area. In inet.4
and udp.4, move the "ip." or "udp." prefix from each entry into the
top-level name at the start of the section, as they are all the same.

Reviewed by: rpokala
Differential Revision: https://reviews.freebsd.org/D35806