file: raise parse error on conflicting section types in strict mode

file: raise parse error on conflicting section types in strict mode

If strict mode is enabled and we're parsing a config file with multiple
sections of the same name but different types, then raise a parse error
to notify the user that the subsequent section declaration would shadow
all prior ones of the same name.

The error would be triggered by a config like that:

  config typeA example
    option test 1

  config typeB example
    option test 2

In such a case, libuci will raise this error:

  uci: Parse error (section of different type overwrites prior section with same name) at line 4, byte 23

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>