os: Use sysctl(KERN_PROC_ARGS) in DetermineClientCmd() on FreeBSD

Open Jan Beich requested to merge jbeich/xserver:freebsd-procargs into master

Inspired by 0ea9b595. At runtime depends on !1047.

/proc has been removed on DragonFly + OpenBSD and deprecated on FreeBSD + NetBSD. One can get char **argv via kinfo_getproc(3) + procstat_getargv(3) or kvm_getprocs(3) + kvm_getargv(3) (API is slightly different than OpenBSD) but since arguments have to be concatenated let's use sysctl(3) + KERN_PROC_ARGS also supported on DragonFly.

CC @manu, @arichardson

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