mgcp: Add new for_each_line macro that also returns empty lines

This patch add the for_each_line macro based on a strline_r()
function (similar to strtok_r()), that is also part of this patch.
This strline_r() function is tolerant with respect to line endings,
it supports CR-only, CRLF, and LF-only and any combinations thereof
(note that a CRLF is always detected as a single line break).

Similar to for_each_non_empty_line (the former for_each_line) where
the 'save' pointer needed to be initialised by a call to strtok_r(),
the new for_each_line macro expects, that the 'save' pointer has been
initialised by a call to strline_r(). Also note, that
for_each_line/strline_r and for_each_non_empty_line/strtok_r may use
the 'save' pointer differently, so calls to them can not be mixed.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/mgcp/mgcp_test.ok b/openbsc/tests/mgcp/mgcp_test.ok
index 5666424..429e0df 100644
--- a/openbsc/tests/mgcp/mgcp_test.ok
+++ b/openbsc/tests/mgcp/mgcp_test.ok
@@ -1,3 +1,16 @@
+line: 'one CR'
+line: 'two CR'
+line: ''
+line: 'one CRLF'
+line: 'two CRLF'
+line: ''
+line: 'one LF'
+line: 'two LF'
+line: ''
+line: 'mixed (4 lines)'
+line: ''
+line: ''
+line: ''
 Testing AUEP1
 Testing AUEP2
 Testing MDCX1