Add helper functions proc_readmem() and proc_writemem().

These helper functions can be used to read in or write a buffer from or to
an arbitrary process' address space. Without them, this can only be done
using proc_rwmem(), which requires the caller to fill out a uio. This is
onerous and results in code duplication; the new functions provide a simpler
interface which is sufficient for most existing callers of proc_rwmem().

This change also adds a manual page for proc_rwmem() and the new functions.

Reviewed by: jhb, kib
Differential Revision: https://reviews.freebsd.org/D4245