Fix/enhance THP integration.

Detect whether chunks start off as THP-capable by default (according to
the state of /sys/kernel/mm/transparent_hugepage/enabled), and use this
as the basis for whether to call pages_nohuge() once per chunk during
first purge of any of the chunk's page runs.

Add the --disable-thp configure option, as well as the the opt.thp
mallctl.

This resolves #541.

src/stats.c

@@ -750,6 +750,7 @@ stats_general_print(void (*write_cb)(void *, const char *), void *cbopaque,
 	OPT_WRITE_BOOL(xmalloc, ",")
 	OPT_WRITE_BOOL(tcache, ",")
 	OPT_WRITE_SSIZE_T(lg_tcache_max, ",")
+	OPT_WRITE_BOOL(thp, ",")
 	OPT_WRITE_BOOL(prof, ",")
 	OPT_WRITE_CHAR_P(prof_prefix, ",")
 	OPT_WRITE_BOOL_MUTABLE(prof_active, prof.active, ",")