implement transparent gzip compression in the webserver

we only compress if applicable (content-type indicates likely compressible),
client supports it, response doesn't already have a content-encoding).

for internal handlers, we always enable compression.  for reverse proxied and
static files, compression must be enabled per handler.

for internal & reverse proxy handlers, we do streaming compression at
"bestspeed" quality (probably level 1).

for static files, we have a cache based on mtime with fixed max size, where we
evict based on least recently used. we compress with the default level (more
cpu, better ratio).

http/atime_bsd.go

@@ -0,0 +1,9 @@
+//go:build netbsd || freebsd || darwin
+
+package http
+
+import "syscall"
+
+func statAtime(sys *syscall.Stat_t) int64 {
+	return int64(sys.Atimespec.Sec)*1000*1000*1000 + int64(sys.Atimespec.Nsec)
+}