blobmsg: implicitly reserve space for 0-terminator in string buf alloc

blobmsg: implicitly reserve space for 0-terminator in string buf alloc

It may not be clear to all users of this API if the provided maxlen argument
refers to the maximum string length or the maximum buffer size.
In order to improve safety and convenience of this API, make it refer to
the maximum string length.

Signed-off-by: Felix Fietkau <nbd@nbd.name>