main/dhcpcd: fix rundir and pidfile
Patchset from opal hart opal@wowana.me via aports #3808:
dhcpcd does not write a pidfile to /run/dhcpcd.pid; this may have been
true in the past, but now it directly writes files (pid, sock, ...) to
the configured rundir. Configure with --rundir=/run/dhcpcd as Gentoo
ebuild for net-misc/dhcpcd does, and direct the initscript to
/run/dhcpcd/pid for pidfile.