net/dhcpcd: Switch keyword from nojail to nojailvnet in rc script

The keyword was added in 2007 following git history, c7e6df59748c. This
is a remnant of the pre-VNET days. dhcpcd is perfectly capable of
running in a VNET jail so relax keyword. This allows dhcpcd to start in
a VNET jail.

PR: 258185
Approved by: 0mp (mentor), philip (mentor)
Differential Revision: https://reviews.freebsd.org/D32959