MFC r321704,r321705,r321706:

r321704:

Add sys/socket.h to SYNOPSIS for libprocstat(3)

sys/socket.h is required for procstat_get_socket_info(3), added in
r221807.

PR: 217884

r321705:

libprocstat(3): fix reference (typo) to procstat_freeenvv in description for
procstat_getargv(3)

PR: 217884

r321706:

libprocstat(3): fix arguments list for procstat_getargv(3) and procstat_getenvv(3)

Neither libcall takes a fourth argument (char *errbuf).

PR: 217884