On Linux, use PR_SET_VMA_ANON_NAME to name memory mappings

[stedolan]

This new Linux prctl call allows a name to be set for anonymous memory mappings. This is useful for debugging (especially performance debugging), as the names show up in /proc/$PID/smaps and are not coalesced with other mappings. (This means that questions like "how much of this process's RSS is OCaml heap" are answerable from smaps).

On a kernel that supports it, here's what part of smaps looks like:

7fa54de00000-7fa54e600000 rw-p 00000000 00:00 0                          [anon:OCaml: minor heap 0]
Size:               8192 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Rss:                4096 kB
Pss:                4096 kB
Pss_Dirty:          4096 kB
Shared_Clean:          0 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:      4096 kB
Referenced:         4096 kB
Anonymous:          4096 kB
LazyFree:              0 kB
AnonHugePages:      4096 kB
ShmemPmdMapped:        0 kB
FilePmdMapped:         0 kB
Shared_Hugetlb:        0 kB
Private_Hugetlb:       0 kB
Swap:                  0 kB
SwapPss:               0 kB
Locked:                0 kB
THPeligible:    1
VmFlags: rd wr mr mw me ac sd 

[mshinwell]

This is good and ok to merge, but a couple of suggestions for potentially useful improvements.