all: cut bloated HTTP-client and tuning options

This is a personal torrent client, not a web client or daemon, so the
rarely-used HTTP-client knobs and redundant tuning flags are gone — flag,
config field, implementation, and tests removed for each. The option
surface drops from 54 to 28.

Web-client cluster removed: --all-proxy (HTTP_PROXY env still works),
load/save-cookies, --referer, -U/--user-agent, --http-user/--http-passwd,
--conditional-get, --remote-time, --check-certificate, --ca-certificate,
--lowest-speed-limit. Pass --header for a one-off auth/cookie/UA header.

Redundant knobs removed: -x and -k (folded into -s), --connect-timeout,
--retry-wait, per-item --max-download-limit/-u, --bt-max-peers,
--bt-stop-timeout, --disable-ipv6, --auto-save-interval, --human-readable,
--stop, -T (a positional .torrent works). follow-torrent and human-readable
are now unconditional defaults.

cli/parse.go

@@ -27,8 +27,8 @@ type Result struct {
 }
 
 // Parse resolves args into options and URIs, applying the layered override
-// order: built-in defaults, then the config file, then proxy
-// environment variables, then the command line (last wins).
+// order: built-in defaults, then the config file, then the command line
+// (last wins).
 func Parse(args []string) (*Result, error) {
 	cmdVals, uris, action, helpTag, err := parseArgs(args)
 	if err != nil {
@@ -48,7 +48,6 @@ func Parse(args []string) (*Result, error) {
 			return nil, err
 		}
 	}
-	applyEnv(opts)
 
 	for name, val := range cmdVals {
 		if err := set(opts, name, val); err != nil {
@@ -225,20 +224,6 @@ func applyConfig(o *Options, path string, explicit bool) error {
 	return sc.Err()
 }
 
-// applyEnv folds the conventional proxy environment variables into all-proxy
-// unless it was already set explicitly.
-func applyEnv(o *Options) {
-	if o.set["all-proxy"] {
-		return
-	}
-	for _, key := range []string{"all_proxy", "https_proxy", "http_proxy"} {
-		if v := os.Getenv(key); v != "" {
-			o.vals["all-proxy"] = v
-			return
-		}
-	}
-}
-
 // set stores a raw value, marking the option as explicitly set. Validation and
 // conversion happen later, once, in finalize(); set only records the string.
 func set(o *Options, name, val string) error {