X-Osmo-IGN: rather parse items by token, not char

Adjust the X-Osmo-IGN parsing to use string tokens instead of parsing single
characters.

Reconsidering the first implementation as a poor choice, rather specify the
format of X-Osmo-IGN as any-length string tokens separated by spaces, which is
more flexible and more future proof.

See also osmo-gsm-manuals If15a88c3b5b40fd1d24ad0f94f3231f678669ab1 which
defines the X-Osmo-IGN format as string tokens, matching this patch.

In mgcp_test, add an unknown X-Osmo-IGN item. Though the output is not checked
by the testsuite.at, running manually shows the error log about the unkown
X-Osmo-IGN item.

Change-Id: Ia6fe5ead4b601931c1bf41b29fc1b237aac37d2c
3 files changed