shell_builtin_read: set cc[VMIN] to 1; lineedit: don't clear c_cc[VINTR]

shell_builtin_read: set cc[VMIN] to 1; lineedit: don't clear c_cc[VINTR]

First change fixes "read -n NUM". Apparently poll() won't report
data availability if cc[VMIN] > 1 until there are at least cc[VMIN] bytes.

function                                             old     new   delta
read_line_input                                     3885    3877      -8
shell_builtin_read                                  1097    1087     -10
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(add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-18)             Total: -18 bytes

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