execve: Add guard pages around execve KVA buffers

[markj]

Summary

This helps ensure that overflows will trigger a panic instead of
silently corrupting adjacent buffers, as happened in SA-26:13.exec.

Extend kmap_alloc_wait() to support allocation of guard pages on both
sides of a KVA allocation. Modify the exec_map setup accordingly.

[kib]

sys/vm/vm_init.c

277

I wonder if exec_map_guard_size should be allowed to set to 0. This might matter on 32bit arches.

[kib]

sys/vm/vm_init.c

277

Or at least we could export exec_map_entr* as RO sysctls to get more visibility there.

[markj]

Add a tunable to control the number of guard pages