scanner: allow strings starting by underscores and dots

scanner: allow strings starting by underscores and dots

POSIX.1-2008 (which is simultaneously IEEE Std 1003.1-2008) says:

"The set of characters from which portable filenames are constructed.

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
a b c d e f g h i j k l m n o p q r s t u v w x y z
0 1 2 3 4 5 6 7 8 9 . _ -"

On top of that it says:

"The <hyphen> character should not be used as the first character of a
portable user name."

This allows a bit more things that NAME_REGEX though, but this still
looks fine to me.

For more info, see:
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_431
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_278

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>