shell: optional support for read -t N.NNN, closes 10101

function                                             old     new   delta
shell_builtin_read                                  1097    1277    +180
dump_procs                                           353     359      +6

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/hush_test/hush-read/read_t0.right

@@ -0,0 +1,3 @@
+><[0]
+><[0]
+><[1]

shell/hush_test/hush-read/read_t0.tests

@@ -0,0 +1,14 @@
+# ><[0]
+echo Ok | { sleep 0.1; read -t 0 reply; echo ">$reply<[$?]"; }
+
+# This would not be deterministic: returns 0 "data exists" if EOF is seen
+# (true terminated) - because EOF is considered to be data (read will not block),
+# else returns 1 "no data".
+## ><[????]
+#true | { read -t 0 reply; echo ">$reply<[$?]"; }
+
+# ><[0]
+true | { sleep 0.1; read -t 0 reply; echo ">$reply<[$?]"; }
+
+# ><[1]
+sleep 0.2 | { read -p IGNORED_PROMPT -t 0 reply; echo ">$reply<[$?]"; }